Agroforest Syst (2015) 89:267–277 DOI 10.1007/s10457-014-9764-8
Soil CO2 flux in an alley-cropping system composed of black locust and poplar trees, Germany T. V. Medinski • D. Freese • C. Bo¨hm
Received: 20 June 2014 / Accepted: 5 November 2014 / Published online: 14 November 2014 Ó Springer Science+Business Media Dordrecht 2014
Abstract Understanding of soil carbon dynamics after establishment of alley-cropping systems is crucial for mitigation of greenhouse CO2 gas. This study investigates soil CO2 fluxes in an alley-cropping system composed of tree strips of black locust (Robinia pseudoacacia L.) and poplar (Populus nigra 9 P. maximowiczii, Max 1) trees and adjacent to them crop strips (Lupinus/Solarigol). Soil CO2 flux was measured monthly over a period from March to November 2012, using a LI-COR LI-8100A automated device. Concurrently with CO2 flux measurements, soil and air temperature, soil moisture, microbial C and hot water-extractable C were determined for the soils nearby soil collars. Root biomass was determined to a depth of 15 cm. In all sampling areas, soil CO2 flux increased from May to July, showing a significant positive correlation with air and soil temperature, which can be a reflection of increase in photosynthesis, and therefore supply of carbohydrates from leaves to rhizosphere, over the warm summer months. A positive correlation between CO2 flux and soil moisture over the warm period indicates an enhancing role of soil moisture on microbial mineralization and root respiration. Average CO2 flux values observed over March–November period did not
T. V. Medinski (&) D. Freese C. Bo¨hm Chair of Soil Protection and Recultivation, Brandenburg University of Technology Cottbus-Senftenberg, KonradWachsmann-Allee 6, 03046 Cottbus, Germany e-mail:
[email protected]
differ significantly between sampling areas, showing 2.5, 3.2, and 2.9 lmol m-2 s-1 values for black locust, poplar and crops, respectively. Significantly higher CO2 flux values over the summer period in trees could be attributed to the higher photosynthetic activity and higher root density compared to crops. Keywords Soil respiration Soil temperature and moisture Microbial C Hot water-extractable C Root biomass
Introduction Soil respiration presents the second largest input of CO2 into the atmosphere (Bohn 1982), and changes occurring in soil respiration relating to land-use change may greatly influence global warming (Kuzyakov and Gavrichkova 2010; Sainju et al. 2008). Generally, scientists recognise two main sources of soil respiration, i.e., root respiration (autotrophic) and microbial respiration (heterotrophic) (Hanson et al. 2000; Kuzyakov 2006). According to Hanson et al. (2000) root and rhizomicrobial respiration may account to 10–90 % of the total CO2 flux. The contribution of root respiration into the total respiration varies between plant species and seasons. Some studies showed that contribution of root-derived CO2 decreases in dormant periods compared to a growing season (Do¨rr and Mu¨nnich 1986; Rochette and Flangan 1997). Shi et al.
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(2012) reported that the largest contribution of the root respiration to the total respiration was observed during growing season and accounted to 46 % of total respiration for oak forest and 60 % for black locust plantation in China. During dormant season it was only 12 % for oak forest and 6 % for black locust plantation. Soil respiration is largely determined by the availability of substrate for roots and microbial respiration (Schimel et al. 1994), environmental condition and land management, which affect microbial activity and rates of organic matter decomposition (Frank et al. 2006; Kuzyakov and Gavrichkova 2010). Among the environmental factors, soil temperature and moisture are considered to have the highest influence on soil respiration (Buchmann 2000; Davidson et al. 2000; Raich and Schlesinger 1992). A number of studies showed an increase in soil respiration with temperature increase (Guner et al. 2010; Jabro et al. 2008; Sainju et al. 2010; Shi et al. 2012). Rainfall and irrigation were also found to have an immediate increasing effect on soil respiration (Jabro et al. 2008; Sainju et al. 2010). Rochette et al. (1992) observed that soil respiration in moist soil was 2-3 times greater than that in drier soils. Sainju et al. (2008) found that irrigation increased CO2 flux by 13 % in North Dacota. This was attributed to the positive effect of moisture increase on microbial activity. Kuzyakov and Gavrichkova (2010), however, explained the correlation between soil respiration and temperature by an increase in photosynthetic activity in warm summer months. According to them, photosynthesis is a key driver of soil respiration, which supplies carbohydrates from leaves to roots and rhizosphere. The increase in temperature and photosynthetic activity are often simultaneous, which can mask the direct effect of photosynthesis. A number of studies showed that soil respiration vary between different land-use systems (Sainju et al. 2008, 2010; Salimon et al. 2004). Cantu-Silva et al. (2010) and Salimon et al. (2004) reported that pasture contributes to more CO2 emissions than forestry systems. Guner et al. (2010) found a higher respiration rates in grassland compared to black locust plantation. Bailey et al. (2009) reported higher respiration rate in strips composed of grass and trees compared to adjacent crops in Missouri. These differences are related to the differences in root density, temperature, moisture availability, quality and quantity of plant residue, which may influence microbial respiration.
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Alley-cropping, an intercropping system composed of crops strips and trees alleys located in between them, has recently become a popular alternative to single crop agriculture in Europe. In addition to crop production, it may produce a woody biomass as a substitute to non-renewable energy resources. It is generally considered that alley-cropping systems enhance CO2 mitigation and soil C sequestration (Nii-Annang et al. 2009; Tsonkova et al. 2012). The information on soil respiration in these systems is, however, limited. This study aims to enhance the knowledge on the loss of OC from the alley-cropping systems. The objective of this study was to determine seasonal changes in CO2 flux in a young temperate alley-cropping system composed of black locust, poplar and crop strips, and to evaluate the influence of the environmental parameters on CO2 flux.
Materials and methods Study area Study was conducted at an alley-cropping site located near town Forst (51°470 N, 14°370 E), Germany. Average air temperature is 8.3 °C, mean annual precipitation is 590 mm. Soil type is characterized as a gleysol. Soil texture is sandy loam. Soil characteristics measured in November 2011 prior to experiment are presented in Table 1. Over the last 50 years site was used for agricultural farming. Prior to establishment of alley-cropping system for the period 2001–2010 various crops were cultivated: winter wheat, barley, silage maize, canola, beets, potatoes, and oats. Alley-cropping system, established in 2010, comprises crop alleys located in-between hedgerows of black locust (Robinia pseudoacacia L.) and poplar clones Max 1 (Populus nigra 9 P. maximowiczii). The total size of the arable field is 40 ha, whereby tree hedgerows have a total area of 5 ha. Each tree strip is 10 m wide and 660 m long. Black locust trees are planted as four double rows, and poplar trees as four single rows. In black locust distance between double rows is 1.8 m, distance between tree rows within a double row is 0.75 m and distance between trees in a row is 0.9 m, amounting to tree density 8,715 trees ha-1 related to tree area. Tree density in poplar comprises 9,804 trees ha-1 related to tree area, with trees planted at 2.5 m distance in between tree rows, and 0.4 m
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Table 1 Soil texture, pH, bulk density, organic carbon and total nitrogen in black locust, poplar, and crop sampling areas at 0–10 cm soil depth layer Sampling area
Clay (%)
Silt (%)
Sand (%)
BDa
pHb
OC (%)
TN (%)
C:N
Black locust
15.5 (1.7)
32.7 (0.2)
51.8 (0.1)
1.5
6.6
1.2 (0.09)
0.11 (0.01)
11 (0.4)
Poplar
10.5 (1.2)
32.2 (1.4)
57.3 (0.6)
1.3
6.5
1.0 (0.18)
0.08 (0.02)
12 (0.6)
Crop
14.4 (1.7)
30.8 (2.5)
54.8 (2.4)
1.2
6.2
1.4 (0.19)
0.12 (0.01)
12 (0.3)
a
Bulk density
b
In 1:2.5 soil: water
distance between trees in a row. Although both black locust and poplar trees were planted in 2010, due to the low rate of survival, poplars were replanted in 2011. The width of crop alleys in-between the tree strips amounted to 96, 48 or 24 m. In March–August 2012 lupines, and in August–November the catch crop mix named Solarigol (composed of Egyptian clover, bitter lupin, linseed, bristle oat, serradella, and squarrose clover) were cultivated in crop alleys.
Methods To evaluate seasonal changes, soil CO2 flux was measured monthly over a period from March to November 2012, except for April and June when no measurements were done. A LI-COR LI-8100A automated device was used for the CO2 flux measurements. All measurements were taken in between 9:00 am and 1:00 pm to reduce variability in CO2 flux due to diurnal changes in temperature (Parkin and Kaspar 2003). Soil polyvinyl chloride collars (11.5 cm tall and 20 cm in diameter) were pushed into the soil with the sharp edge to a depth of 3 cm. A mark was placed in the plot for the soil flux to be measured at the same place though the study. Soil surface was cleaned from litter immediately before placing the soil collar. Measurements were done at three investigation areas: black locust and poplar tree strips and adjacent crop alleys. Each sampling area had three replicate plots located along tree strips and crop alleys. Crop plots were located at 12 m distance from tree strips. Plots have comparable climatic and soil characteristics. Three soil cores were inserted at each replicate plot at 2 m distance from each other. At trees, soil cores were inserted inbetween tree rows at 1 m distance from the tree rows. Two measurements (3 min each with CO2 flux recorded every 1 s) with 2 min interval in between measurements
were done at each soil collar. Average values were calculated from these values for each soil collar. Simultaneously with CO2 flux measurements, soil and air temperature were recorded by means of thermometer to a depth of 10 cm within 10 cm distance from each collar. Three soil samples were collected nearby each soil collar from 0 to 10 cm depth, bulked together, mixed thoroughly to present one composite sample for each soil core, and used for soil moisture, microbial C, hot water-extractable organic carbon (HWC), and hot water-extractable nitrogen (HWN) analyses. Samples were stored in sealed bags at 0 °C prior to soil moisture and microbial analyses. Samples for HWC and HWN were air-dried and 2 mm sieved prior to analyses. Soil moisture content was determined by drying soil samples in an oven at 105 °C for 72 h, and was expressed on a dry mass basis. Microbial C was measured by microwave irradiation method according to Islam and Weil (1998), and HWC and HWN according to Ko¨rschens et al. (1990) using a TOC analyser (Schimadzu). Root biomass was determined to a depth of 15 cm in July. For this 5 soil cores (7 cm diameter and 15 cm long) were taken at each replicate plot. Soils from soil cores were bulked together. Coarse roots were removed by hands. Smaller roots were removed by repeated wet sieving and using forceps. Coarse and fine roots were bulked together, dried at 50 °C for a few days till constant weight, and weighed. Air temperature (°C) and precipitation (mm) were obtained from a meteorological station located in the middle of 96 m wide crop alley. The data for the March and April were not obtained because of technical problems with the weather station in these months. Statistical analyses Data was analyzed for normality by Shapiro–Wilk Test. As not all the data were normally distributed, a
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Mann–Whitney U Test was done to compare the mean differences between investigation areas in measured properties. Differences were considered significant at p \ 0.05. Spearman correlation coefficients were determined to test correlations between variables. All analyses as well as descriptive data statistics (mean, standard deviation) were done with SPSS program (version 21, 2012, IBM). The regression lines, showing the highest goodness of fit (R2), and equations for the relationships between variables in Figs. 3, 5 were obtained with Excel 2007 program.
September, followed by a gradual decrease over the October–November period to 2.9 lmol m-2 s-1. CO2 flux in crops was significantly (p \ 0.05) lower than in the trees over the March–August period. In September– October period, crops had significantly (p \ 0.05) greater CO2 flux compared to the trees. Monthly sum of precipitation exhibited two peaks: a sharp increase over May–July period from 32 to 210 mm, followed by a decrease to 28 mm in August; and another increase in September to 68 mm, followed by a decrease in October–November period to 34 mm (Fig. 2). Mean monthly air temperature increased over March–July period from 4 to 18 °C, followed by a decrease from to 7 °C over September–October period. Mean soil temperature, calculated for the whole experimental period (March–November), was slightly although not significantly lower in black locust (11 °C) compared to poplar (12 °C) and crops (12 °C) (Table 2). Soil temperature increased over the March–August period from 7.3 to 15.4 °C in black locust, 7.3–18.8 °C in poplar, and 7.3–18.2 °C in crops, followed by a decrease over the August– November period to 5.9 °C in black locust, 6.0 °C in poplar and 5.3 °C in crops (Fig. 1b). Both soil and air temperature were positively correlated with CO2 flux, showing Spearman correlation coefficients r = 0.64 and r = 0.61 (p \ 0.01), respectively (Table 3). Average across measurement dates soil moisture was slightly lower in poplar (11 %) compared to black locust (13 %) and crops (13 %) (Table 2). Soil moisture decreased over March–August period from 22 to 6 % in black locust, 18–5 % in poplar and
Results The mean CO2 fluxes averaged for the whole experimental period (March–November) did not show significant differences between sampling areas, comprising 2.5, 3.2 and 2.9 lmol CO2 m-2 s-1 for black locust, poplar and crops, respectively (Table 2). There were, however, differences between sampling areas at different months. In black locust and poplar, soil CO2 flux increased from 1.6 and 1.1 lmol m-2 s-1 in March to 5.1 and 6.5 lmol m-2 s-1 in July, respectively, and then gradually decreased to 1.6 and 1.5 lmol m-2 s-1 in November (Fig. 1a). In July, September and October, CO2 flux was significantly (p \ 0.05) greater in poplar compared to black locust. While in the other months, no significant differences were found between tree species. In crops, two peaks in soil CO2 flux were observed: an increase from 0.3 to 3.7 lmol m-2 s-1 over the March– July period, followed by a decrease to 2.5 lmol m-2 s-1 in August; and another increase to 3.9 lmol m-2 s-1 in
Table 2 CO2 flux and measured properties in black locust, poplar and crop sampling areas Black locust
Poplar
Crop
CO2 flux (lmol m-2s-1)
2.5 (1.1)
3.2 (2.1)
2.9 (1.5)
Soil T (°C)
11 (4)
12 (5)
12 (5)
Air T (°C)
14 (5)
15 (7)
14 (6)
13 (6)ab
11 (5)a
13 (3)b
Soil moisture (gravimetric %) -1
Microbial C (mg kg )
241 (91)
236 (71)
-1
618 (162)
-1
a
HWC (mg kg ) HWN (mg kg )
96 (19)
HWC:HWN
6.7 (l.l) -1
Roots biomass (Mg ha )
a
ab
2.0 (0.5)
a
275 (70)
489 (147) 73 (20)
b
b
618 (108)a 102 (22)a
7.1 (0.9)
a
6.4 (1.0)b
2.2 (0.3)
a
0.8 (0.0)b
Data are averaged for the whole sampling period (March–November 2012) and presented as mean and standard deviation values a,b
Different letters indicate significant difference between sampling areas (Mann–Whitney U test, p \ 0.05)
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Fig. 1 Seasonal changes in a soil CO2 flux, b soil temperature, c soil moisture, d microbial C, e hot waterextractable carbon; and f hot water-extractable nitrogen in black locust, poplar and crops. Data are presented as mean values for each sampling area. Vertical lines indicate standard deviation. a, b, c different letters indicate significant differences between sampling areas for each particular month (p \ 0.05, Mann–Whitney U Test)
18–9 % in crops, followed by an increase over September–November period from 9 to 14 % in black locust, 8–14 % in poplar, and a slight decrease from 14 to 12 % in crops (Fig. 1c). Poplar had significantly (p \ 0.05) lower soil moisture compared to black locust and crops in July–August, while crops had significantly higher soil moisture compared to trees in August–October period. Soil moisture was negatively correlated with soil temperature (r = -0.30, p = 0.018) and with air temperature (r = -0.37, p \ 0.01) over March–November period (Table 3). The relationships between the soil moisture and CO2 flux were not significant for the March–November period (Table 3). While for May–October period, a significant positive correlation (r = 0.553, p \ 0.001) was observed between the soil moisture and CO2 flux (Fig. 3). Average across measurement dates HWC was significantly (p \ 0.05) higher in black locust (618 mg kg-1) and crops (618 mg kg-1) compared
to poplar (489 mg kg-1) (Table 2). HWC showed seasonal variations, with significantly (p \ 0.05) lower amounts in poplar compared to black locust and crops in March, May and October (Fig. 1e). In November, HWC increased in both tree species compared to the other months, showing significantly (p \ 0.05) higher values in black locust (961 mg kg-1) compared to poplar (702 mg kg-1) and crops (572 mg kg-1). HWC was significantly negatively correlated with soil temperature (r = -0.291, p = 0.021) Similar to HWC, average across measurement dates HWN was also significantly (p \ 0.05) higher in black locust (96 mg kg-1) and crops (102 mg kg-1) compared to poplar (73 mg kg-1) (Table 2). It showed some seasonal variability with significantly (p \ 0.05) lower values in poplar compared to crops in March, May, September and October, and compared to black locust in September and November (Fig. 1f). In November, HWN was significantly higher in black locust
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poplar, and 343–333 mg kg-1 in crops (Fig. 1d). Crops showed a trend of higher microbial C compared to black locust, with significantly (p \ 0.05) higher values in July and August. Microbial C was significantly negatively correlated with CO2 flux (r = -0.37, p \ 0.01), soil temperature (r = -0.45, p \ 0.001) and air temperature (r = -0.48, p \ 0.001), and positively correlated with HWC (r = 0.58, p \ 0.001), HWN (r = 0.50, p \ 0.01), and HWC: HWN ratio (r = 0.26, p = 0.039) (Table 3; Fig. 5). Root biomass was significantly (p \ 0.05) higher in poplar and black locust compared to crops, comprising 2.2, 1.96 and 0.8 Mg ha-1, respectively (Table 2).
Discussion Relationships between soil flux and measured parameters Fig. 2 Mean monthly precipitation and air temperature in study site over the January–November 2012 period. The average values are calculated for a 30-days period prior to analyses. Vertical lines indicate standard deviation
(124 mg kg-1) compared to poplar (88 mg kg-1) and crops (88 mg kg-1). HWN was significantly positively correlated with HWC (r = 0.900, p \ 0.0001) (Table 3). Average across measurement dates HWC:HWN ratios were significantly (p \ 0.05) lower in crops compared to poplar, comprising 6.4, 7.1, and 6.7 for crops, poplar and black locust, respectively (Table 2). The HWC: HWN ratios ranged between 5.4 and 6.8 over May–September period, and 7.3–8.2 over March, and October–November period (Fig. 4). They were significantly negatively correlated with CO2 flux (r = -0.573, p \ 0.0001), soil temperature (r = -0.632, p \ 0.0001) and air temperature (r = -0.621, p \ 0.0001) (Table 3). Microbial C averaged across measurement dates was slightly but insignificantly higher in crops (275 mg kg-1) compared to black locust (241 mg kg-1) and poplar (236 mg kg-1) (Table 2). It showed a decrease over March–May period from 245 to 146 mg kg-1 in black locust, 228–178 mg kg-1 in poplar and 262–216 mg kg-1 in crops; followed by a lower values period over May–August, and an increase over September–October period: with a range of values 310–373 mg kg-1 in black locust, 297–341 mg kg-1 in
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Soil CO2 flux followed a trend reported in a number of studies, showing an increase over March-July period, higher values over summer period and decline in autumn (Jabro et al. 2008; Jinsong et al. 2008; Sainju et al. 2010). Similar pattern was reported for black locust in Turkey by Guner et al. (2010) and in China by Shi et al. (2012). The increase in soil respiration coincided with the increase in soil and air temperature in summer period, showing a positive correlation between these variables. Soil and air temperature are seen as the main environmental factors which may influence soil respiration. A number of studies reported a positive correlation between CO2 flux and soil and air temperature (Cantu-Silva et al. 2010; Jabro et al. 2008; Jinsong et al. 2008; Lee and Jose 2003). This is because high CO2 flux usually occurs in the summer, when soil temperature is higher and soil water content and availability of carbohydrates are adequate for microbial activity, and low CO2 flux occurs in winter when soil biological activity is minimal due to the low soil temperature (Bajracharya et al. 2000; Sainju et al. 2008). A better correlation between CO2 flux and soil temperature compared to air temperature was in accordance with Sainju et al. (2008). It highlights the higher influence of soil temperature on microbial mineralisation of soil organic matter. A positive correlation observed between CO2 flux and soil and air temperature, may be attributed to the
Agroforest Syst (2015) 89:267–277 Table 3 Spearman correlation coefficients (r) for the relationships between variables (n = 63) for the data collected across all sampling areas over the March–November 2012 period
273
Soil T
Air T
Moisture
Microbial C
HWC
HWN
HWC:HWN
-0.103
-0.365**
-0.573**
CO2 flux r
0.643**
0.613**
p
0.000
0.000
-0.238
-0.027
0.421
0.003
0.061
0.833
-0.296*
-0.447**
-0.291*
0.018
0.000
0.021
0.367
-0.371** 0.003
-0.478** 0.000
-0.239 0.060
-0.074 0.564
0.000
Soil T r
0.951**
p
0.000
-0.116
-0.632** 0.000
Air T r p
-0.621** 0.000
Moisture r
0.112
0.125
0.068
-0.016
p
0.382
0.328
0.595
0.900
Microbial C r
0.577**
0.500**
0.260*
p
0.000
0.000
0.039
r
0.900**
0.174
p
0.000
0.172
HWC p indicates level of significance ** Correlation is significant at the 0.01 level (2-tailed)
HWN r
-0.086
* Correlation is significant at the 0.05 level (2-tailed)
p
0.502
Fig. 3 Relationships between soil moisture and CO2 flux observed across all sampling areas over the May–October period. The R2 and equation of the best-fit exponential regression line fitted through data points (n = 45) is presented. Spearman correlation coefficient r = 0.553, p \ 0.001
Fig. 4 Relationships between ratios of HWC : HWN and soil CO2 flux at each particular month over the March–November 2012 period in black locust and poplar replicate plots (n = 6)
increase in photosynthetic activity in summer period, at the time of rapid vegetation growth. According to Kuzyakov and Gavrichkova (2010), photosynthesis is the main driving force of CO2 flux, supplying carbohydrates from leaves to rhizosphere. Simultaneous increase in soil temperature and photosynthesis
activity in summer may mask the driving role of photosynthesis on CO2 flux. Contrary to the numerous findings reporting positive correlation between soil moisture and soil respiration (Salimon et al. 2004; Jabro et al. 2008; Sainju et al. 2008, 2010), our study showed a lack of correlation for the
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Fig. 5 Relationships between microbial C and a hot waterextractable C, and b hot water-extractable N observed across all sampling areas over the March–November period. The R2 and equation for the best-fit linear regression line fitted through data points (n = 63) is presented
values obtained over the whole measurement period (March–November). In contrast, soil moisture showed an opposite pattern to CO2 flux, showing higher values in early spring and autumn and lower values in summer. This was similar to findings of Davidson et al. (1998) for the mixed hardwood forest (red oak, red maple) in Massachusetts. They also observed opposite patterns for soil moisture and respiration for the whole year sampling period, with an increase in soil respiration over the drier spring-summer period, and decline in soil respiration in autumn and winter periods, which coincided with the temperature decrease and soil moisture increase. In a period of low air temperature and high moisture inputs with snow melting in early spring and higher rainfall in autumn, low evaporation could have resulted in higher soil water content. Despite it is generally considered that soil moisture should promote microbial activity, and therefore promote mineralisation and release of CO2 (Van Gestel et al. 1993), the low soil temperature and lack of photosynthesis in March and November restricted soil respiration. Furthermore, excessive amount of soil moisture could have resulted
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in soil sealing, and filling in soil pores, trapping CO2 and reducing CO2 flux. We suggest that the effect of moisture availability should be the most prominent in dry periods. Studies of Jabro et al. (2008) and Sainju et al. (2008; 2010) conducted in dry North Dakota showed that irrigation and precipitation caused an outburst of microbial activity and C mineralisation, resulting in substantial increase of CO2 flux. In our study, when cold, non-photosynthetic periods (March and November) were excluded from the correlation analyses, a positive correlation was found between CO2 flux and soil moisture in May–October. Similarly, in the study of Davidson et al. (1998), a positive correlation between soil moisture and respiration was observed in the warm summer-autumn period, when rain after draught in August caused an increase in soil respiration. This is probably because soil temperature was adequate (not-restricting), and increase in soil moisture enhanced microbial and rhizosphere respiration. Hot water-extractable carbon is considered to present labile, easily extractable C compounds, available for microbial organisms, which should enhance microbial respiration (Ghani et al. 2003). In our study, HWC was the highest in November, when CO2 flux and soil temperature were low. This could be related to the higher litter returns, which could have increased the amount of easily leached and decomposed C compounds in soil. Higher HWC: HWN ratios (7–10) in March and in October–November period showed the lower levels of decomposition of fresh litter residue, compared to the more decomposed OM with a narrower HWC: HWN ratio (5–7) in summer. This can explain the negative correlation observed between HWC: HWN and soil and air temperature, and CO2 flux. The higher HWC in black locust and crops compared to poplar over March–October period, may be attributed to the higher litter returns in the previous years. Although both black locust and poplar were planted in the same year, most of the poplar trees died in the first year and had to be replanted in the following year, resulting in the lower litter returns compared to black locust. In crops, straw was left on soil after harvesting, which could have increased OM, and therefore HWC compared to poplar. In addition to HWC, soil organic matter is a main source of HWN in soils. This explains a correlation observed between HWC and HWN. Higher HWN in black locust and lupines may be attributed to N-fixation by these species, as N-fixing plants have N-rich litter which may increase soil N levels. In
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November, despite an increase in HWC related to the higher litter returns in trees, HWN did not increase in poplar. This probably relates to the lower N levels in poplar litter, and high C:N ratio, which could have resulted in N immobilisation. Substrate availability plays an important role for microbial growth (Kuzyakov and Gavrichkova 2010). The higher input of the readily available C from litter residue in November in combination with high moisture availability and non-restricting temperature conditions could have immediately increased microbial biomass. This may explain a positive correlation observed between microbial C, HWC, and HWN, and negative correlation between microbial C and soil and air temperature and CO2 flux. Despite microbial respiration is one of the main sources of CO2 flux, the relationships between these two variables may be not straightforward. Contribution of microbial respiration into the total CO2 flux may vary between different seasons. In the growing season root respiration may account for the biggest input to the CO2 flux. While in dormant season, contribution of rootderived CO2 decreases, and microbial respiration input into the total CO2 flux was found to increase (Do¨rr and Mu¨nnich 1986; Rochette and Flangan 1997; Shi et al. 2012). We did not partitioned between microbial and root respiration, and further studies are warrant to validate this pattern. The microbial C determined in our study is a measure of microbial biomass, and shows C trapped in microbial bodies. Higher microbial C does not necessarily means higher respiration, as microbes may be inactive. For example, Lai et al. (2014) showed that with similar microbial biomass, microbial respiration may significantly vary between the soils under canopy and in open spaces. Therefore, we suggest that microbial respiration is a better measure within soil respiration studies than microbial biomass. In order to exclude seasonal factors which may modify the effect of microbial respiration on CO2 flux, relationships between CO2 flux and microbial C should be investigated at a shorter time intervals or under controlled conditions. It would also be of interest to investigate microbial species richness and the role of different microbial species in CO2 flux. Differences in CO2 flux between sampling areas The values observed for CO2 flux were similar to Shi et al. (2012), who reported mean annual respiration in
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black locust plantation equal to 2.76 lmol m-2 s-1, which decreased to 1.16 lmol m-2 s-1 in dormant season (October–December). Similarly, in our study, mean CO2 flux over March–November period was 2.5 and 1.3–1.6 lmol m-2 s-1 in October–November. The greater CO2 flux in trees compared to crops over the summer period were in accordance with Peichl et al. (2006) and Bailey et al. (2009), who found greater soil CO2 flux in agroforestry tree strips compared to adjacent crop alleys. This could be attributed to the higher photosynthetic activity as well as greater root respiration. Trees have more aboveground biomass, and may assimilate more CO2, which is later transported below ground to the roots, and may result in greater roots and rhizosphere respiration. Root density is one of the most important factors affecting soil respiration (Ben-Asher et al. 1994). As found by Shi et al. (2012), in black locust plantations, root respiration may account up to 60 % of the total CO2 flux in growing summer season, when the supply of readily available photosynthesises to roots results in immediate increase in root respiration. The higher soil respiration in poplar compared to black locust in July, September, and October in our study could be related to the higher root density (although not significantly), as well as to a better exposure to the sunlight and higher soil temperature. A removal of weeds in between rows of poplar trees resulted in better exposure to sunlight, and in increase of soil temperature. While in black locust, wide crowns of trees and understory vegetation, which was not cleared, shaded soils and prevented sunlight penetration. Shading could also increase soil moisture in black locust compared to poplar, where better exposure to sunlight, could have resulted in greater evaporation of soil moisture. Despite lower soil moisture in poplar, soil moisture conditions were probably not restricting, and greater soil temperature could have enhanced roots and rhizomicrobial respiration. In crops, CO2 flux was characterised by two peaks with increase in July, and in September, reflecting two vegetative periods. A crop mix Solarigol was seeded in August just after harvesting of lupines. By the time of CO2 flux measurements in September, crop plants were about 50 cm high. This was a period of their rapid growth, and intensive photosynthetic activity, resulting in intensive assimilation of CO2, rapid transfer of C assimilates to the roots, and higher root respiration. As reported by Joslin et al. (2001), period of the intense root
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growth should be accompanied by increase in root respiration. In contrast to crops, photosynthetic activity and root respiration in trees decline in autumn (Do¨rr and Mu¨nnich 1986; Rochette and Flangan 1997). As found by Shi et al. (2012), root respiration in black locust plantation declined in autumn and accounted to only 6 % of the total CO2 flux. This is because at the end of growing season prior to defoliation, plants translocate C assimilates to the roots for a storage over the dormant period. These assimilates may also be used for the root respiration, but root respiration is lower than in summer (Kuzyakov 2006). Soil tillage prior to crop seeding could have also increased emission of CO2 from the soil. As reported by Sainju et al. (2008, 2010), tillage increased CO2 flux compared to no-tilled trees alleys in western North Dakota. This is because soil tillage disrupts soil aggregates, incorporates plant residues into soil, increases aeration and diffusion of CO2 through the porous media, as well as microbial mineralisation of organic matter. Soil tillage could have also increased soil moisture content, by increasing soil pore spacing and rain water infiltration, compared to compacted soils in tree strips. The higher moisture content observed in August–October period in crop soils could further enhance rhizomicrobial respiration. Despite a seasonal variation in CO2 flux between the different sampling areas, average CO2 flux for the whole study period showed no significant differences between sampling areas. A greater C loss with soil respiration in trees in summer period may be compensated by greater C assimilation and storage in woody biomass, and the greater respiration from crops after tillage in autumn. In a long run, a lower soil disturbance in trees may promote soil aggregation and sequestration of OC within soil aggregates. This, in conjunction with the greater C assimilation in aboveground tree biomass, indicates a potential of trees hedgerows to sequester C. The agroforestry system in our study is still young. With trees getting older, changes in soil respiration may occur. By the end of the first rotation period (after 4 years) black locust and poplar trees may reach more than 6 m in height, and trees canopies would spread wider. This may change microclimatic conditions in tree hedgerows and adjacent crop strips. As reported by Martius et al. (2004), canopy closure strongly affected soil temperature, moisture, and macrofauna. Well developed canopy may protect soil macrofauna from
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high temperatures and draught stress. Trees hedgerows may shade adjacent crop areas, which would decrease soil temperature, reduce soil evaporation and increase soil moisture content, as well as affect photosynthetic activity of crop strips. Changes in microclimatic conditions and root densities with increasing trees age may have a significant influence on CO2 flux.
Conclusion An increase in soil CO2 flux over the warm summer period positively coincided with the changes in soil and air temperature. This pattern may be a reflection of the higher photosynthetic activity in summer, which supplies C compounds to roots and increases rhizomicrobial respiration. A positive correlation observed between soil moisture and CO2 flux over the warm period, indicates that when the conditions are not restricted by the low temperatures or lack of photosynthesis, the increase in soil water content may enhance soil respiration. Different seasonal peaks in CO2 flux in trees hedgerows and crops strips highlight the role of vegetation period for soil respiration. Despite a seasonal variation in CO2 flux, average CO2 flux for the whole study period showed no significant differences between the young trees hedgerows and adjacent crop strips. A greater C loss with soil respiration from trees hedgerows in summer may be compensated by greater C assimilation and storage in woody biomass, and the greater respiration from crops after tillage in autumn. A longer-time investigation at different locations and soil conditions are warrant to validate this pattern, and to assess changes in microclimate, and root density with the increasing age of trees hedgerows, and their influence on soil respiration. Acknowledgments This study was supported by the German Federal Ministry of Food, Agriculture and Consumer Protection (Project ‘‘AgroForstEnergie II’’, project number: 22000312), and the German Federal Ministry of Education and Research (Project ‘‘INKA BB’’, project number: 01LR0803D). The authors extend their grateful thanks to the farm company AG Forst e.V. for allowing measurements and soil sampling, as well as to technical assistants Sebastian Heller and Katja Westphal, for the help with field measurements and laboratory analyses.
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