Plant Soil DOI 10.1007/s11104-014-2077-x
REGULAR ARTICLE
Fertilization enhancing carbon sequestration as carbonate in arid cropland: assessments of long-term experiments in northern China X. J. Wang & M. G. Xu & J. P. Wang & W. J. Zhang & X. Y. Yang & S. M. Huang & H. Liu
Received: 28 August 2013 / Accepted: 28 February 2014 # Springer International Publishing Switzerland 2014
Abstract Aims Soil inorganic carbon (SIC), primarily calcium carbonate, is a major reservoir of carbon in arid lands. This study was designed to test the hypothesis that carbonate might be enhanced in arid cropland, in association with soil fertility improvement via organic amendments. Methods We obtained two sets (65 each) of archived soil samples collected in the early and late 2000’s from three long-term experiment sites under wheat-corn cropping with various fertilization treatments in northern China. Soil organic (SOC), SIC and their Stable 13C
compositions were determined over the range 0– 100 cm. Results All sites showed an overall increase of SIC content in soil profiles over time. Particularly, fertilizations led to large SIC accumulation with a range of 101– 202 g C m−2 y−1 in the 0–100 cm. Accumulation of pedogenic carbonate under fertilization varied from 60 to 179 g C m−2 y−1 in the 0–100 cm. Organic amendments significantly enhanced carbonate accumulation, in particular in the subsoil. Conclusions More carbon was sequestrated in the form of carbonate than as SOC in the arid cropland in north-
Responsible Editor: Eric Paterson. X. J. Wang (*) : J. P. Wang State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi, Xinjiang 830011, China e-mail:
[email protected] X. J. Wang College of Global Change and Earth System Science, Beijing Normal University and Joint Center for Global Change Studies, Xinjiekouwai Street No.19, Haidian District, Beijing 100875, China M. G. Xu : W. J. Zhang Ministry of Agriculture Key Laboratory of Crop Nutrition and Fertilization, Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, China J. P. Wang Graduate University of Chinese Academy of Sciences, Beijing 100049, China
X. Y. Yang College of Natural Resources and Environment, Northwest Agricultural and Forestry Science and Technology University, Yangling, Shaanxi 712100, China
S. M. Huang Institute of Plant Nutrition, Resources and Environment, Henan Academy of Agricultural Sciences, Zhengzhou, Henan 450002, China
H. Liu Institute of Soil and Fertilizer and Agricultural Sparing Water, Xinjiang Academy of Agricultural Sciences, Urumqi 830091, China
Plant Soil
ern China. Increasing SOC stock through long-term straw incorporation and manure application in the arid and semi-arid regions also enhanced carbonate accumulation in soil profiles. Keywords Soil carbonate . Stable 13C composition . Carbon sequestration . Fertilization . Cropland . Arid region
Introduction Soil organic carbon (SOC) and inorganic carbon (SIC) are important reservoirs of carbon on the global lands. Soil organic carbon, as a key index for soil fertility and carbon sequestration, has gained recognition (Lal 2001, 2004). The estimated global SOC pool for the upper 100 cm has a relatively narrow range, i.e., 1220–1576 Pg (Eswaran et al. 2000). In contrast, there is a large discrepancy in the estimated global SIC pool, which ranges from less than 700 Pg to more than 1700 Pg (Eswaran et al. 2000), and SIC has received much less attention despite its potential for carbon sequestration and climate mitigation (Eshel et al. 2007; Lal and Kimble 2000; Manning 2008). Soil inorganic carbon, primarily calcium carbonate, is the most common form of carbon in soils of arid and semi-arid regions. More than 35 % of Earth’s land surface is characterized as either arid or semi-arid, where SIC stock is 1–9 times higher than SOC stock (Scharpenseel et al. 2000; Schlesinger 1982). Therefore, accurately estimating SIC at all scales is essential to evaluate the role of soils in the global carbon cycle. Moreover, attempts to decrease the atmospheric CO2 concentration require better understanding of carbon sequestration as all forms in various systems, including carbonate in arid and semi-arid lands (Monger et al. 2009; Zheng et al. 2011). The SIC pool consists of two major components: the lithogenic carbonate (LIC) and pedogenic carbonate (PIC). The former originates as detritus from parent materials, mainly limestone, whereas the secondary carbonate is formed by the dissolution and re-precipitation of LIC or through dissolution of carbon dioxide (CO2) into HCO3−, then precipitation with Ca2+ and/or Mg2+ originating from non-LIC minerals (e.g., silicate weathering). Thus, PIC formation in soils could lead to carbon sequestration (Monger and Gallegos 2000). Recent reports (Wohlfahrt et al. 2008; Xie et al. 2009) of significant CO2 uptake (>100 g C m−2 yr−1) in deserts
have raised some questions, e.g., where does the carbon go (Stone 2008). One hypothesis was made that the absorbed CO2 would be precipitated as calcium carbonate. Affirmation of this hypothesis requires evidence of PIC accumulation. However, field data necessary to quantify carbon sequestration as carbonate have been lacking although limited studies seem to indicate that PIC accumulation is extremely low in most parts of the arid and semi-arid regions (Schlesinger et al. 2009). The Chinese National Soil Fertility and Fertilizer Efficiency Monitoring Network was established in 1990 across China, to study the effects of different fertilization managements on agricultural productivity and soil fertility. There have been numerous studies using data collected from the network to evaluate carbon sequestration as SOC under various fertilization treatments (e.g., Cong et al. 2012; Shen et al. 2007; Zhang et al. 2010), which demonstrated widespread increases in SOC stock as a results of fertilization in northern China. Given that there was evidence of more carbon sequestrated in the form of PIC than as organic matter in arid regions (Landi et al. 2003), we hypothesized that there would be significant carbonate accumulation in arid croplands, in association with the increases of SOC. To test this hypothesis, we selected three sites under arid and semi-arid climatic conditions across northern China. Each had the same experiment design with similar fertilization treatments. We obtained 130 archived soil samples collected from different years under various fertilization managements, and analyzed SOC and SIC contents and their Stable 13C compositions in the 0– 100 cm soil profiles. The two objectives of this work are to study the soil carbonate dynamics in arid cropland, and to evaluate impacts of fertilization management on carbonate accumulation in soil profile.
Materials and methods Description of the long-term experiment sites There are three long-term experiment (LTE) sites in the network under arid (Urumqi) and semi-arid (Yangling and Zhengzhou) climatic conditions in northern China (Table 1). Annual average temperature and precipitation are lower in Urumqi (7.7 °C and 299 mm) than in Yangling and Zhengzhou (13–14.5 °C and 550– 632 mm). Annual evaporation varies from 993 mm to 2,570 mm. Irrigation is usually applied during the crop’s
Plant Soil Table 1 The characteristics of long-term experiment sites in northern China
a
Bulk density
Properties
Unit
Urumqi
Yangling
Zhengzhou
Latitude
N
43°59′26″
34°17′51″
34°47′25″
Longitude
E
87°46′45″
108°00′48″
113°40′42″
Altitude
m
600
525
59
Annual mean temp.
°C
7.7
13.0
14.5
Annual precipitation
mm
299
550
632
Annual evaporation
mm
2,570
993
1,450
Soil classification (FAO)
Haplic Calcisol
Calcaric Cambisol
Parent material
Limestone
Calcaric Regosol Loess
Soil pH
8.1
8.6
8.3
River Alluvium
Initial SOC (0–20 cm)
g kg−1
8.8
7.4
6.7
Initial TN (0–20 cm)
g kg−1
0.87
0.83
0.67
BDa (0–20 cm)
g cm−3
1.21
1.35
1.41
BD (20–40 cm)
g cm−3
1.35
1.56
1.44
BD (40–60 cm)
g cm−3
1.48
BD (60–80 cm)
g cm−3
1.43
BD (80–100 cm)
g cm−3
1.36
Cropping system
Mono
Double
Crop rotation
Corn-wheat-wheat
Wheat-corn
Wheat-corn
468
196
400
Plot size
growing seasons. All three sites have calcareous soils, with illite as one of main minerals. Chlorite is also dominant at Urumqi, and smectite at the other sites. Initial SOC content was in a range of 6.7–8.8 g kg−1, initial total nitrogen 0.67–0.87 g kg−1, and soil pH 8.1– 8.6. Bulk density was measured for the 0–40 cm at the Zhengzhou and Urumqi sites, and the 0–100 cm at Yangling. The crops at these sites are corn (Zea mays L.) and wheat (Triticum Aestivium L.). The Urumqi site has a mono-cropping system whereas the other two sites have a double-cropping system. The mono-cropping system has a rotation of corn-wheat-wheat, with corn seeded during late April to early May and followed by spring wheat (seeded in mid-April) then winter wheat (seeded in late September in the same year). The double cropping systems have a rotation of summer corn (seeded in late April to early May) and winter wheat (seeded in October). While there are no replicates at all three sites (note: most LTEs in the network have no replicates), experiment plots are large enough (196–468 m2) to be representative. In addition, all sites had 2–3 years of land preparations (e.g., cultivation with tillage) to eliminate spatial variations in soil conditions prior to experiments. A number of studies have carried out to
m2
Double
evaluate the impacts of fertilization treatments on SOC dynamics using data collected from the network (Cong et al. 2012; Shen et al. 2007; Zhang et al. 2010). We obtained two sets of archived soil samples: one set collected in 2009 and the other set from the early 2000s (2001 for the Urumqi site and 2002 for the other two sites). Five treatments were included: (1) no fertilization (control), (2) mixed mineral nitrogenphosphorus-potassium fertilization (NPK), (3) NPK fertilization with straw incorporation (NPKS), (4) NPK fertilization with manure application (NPKM), and (5) 50–100 % higher application rates of minerals and manure than the NPKM treatment (hNPKM). Some soil samples were missing for the NPKS treatment at the Urumqi site, and there were contamination in some soils collected under hNPKM from the Zhengzhou site. Thus, we excluded the NPKS treatment for the Urumqi site and hNPKM for the Zhengzhou site in this study. The mineral nitrogen, phosphorus and potassium fertilizers were urea, calcium superphosphate, and potassium chloride (or potassium sulfate), respectively. For the NPKS treatment, all above-ground materials (except the grains) from one crop (corn at the Yangling and Zhengzhou sites) were incorporated into topsoil each year. Fertilization rates and manure types, summarized in
Plant Soil
Table 2, were based on local agricultural practices (Yang et al. 2012; Zhang et al. 2010).
Soil sampling and analyses Soil samples were collected from five layers (0–20, 20– 40, 40–60, 60–80, and 80–100 cm) during September– October in 2002 at Yangling and Zhengzhou (2001 at Urumqi) and 2009 at all three sites. There were 5–10 cores (5-cm-diam) of soil in each layer, randomly sampled in each plot, air dried and thoroughly mixed. Representative sub-samples were crushed to 0.25 mm for SOC and SIC measurements that were carried out using a CNHS-O analyzer (Model EuroEA3000). For SOC measurement, 20 mg soil was pretreated with 10 drops of H3PO4 for 12 h to remove carbonate. The pretreated sample was combusted at 1,020 °C with a constant helium flow carrying pure oxygen to ensure completed oxidation of organic materials. Production of Table 2 Application rates of nitrogen (N), phosphorus (P), and potassium (K) for each growing season under different treatments Treatments Urumqi
Yangling
Zhengzhou
Maize/wheat Maize Wheat
Maize Wheat
Nitrogen (kg N ha−1) CK
0
0
0
0
0
NPK
242
188
165
188
165
NPKS
–
188
165+43a
188
123+42a
NPKM
85+240a
188
50+115a
188
50+115a
188
74+173a
hNPKM 152+360a −1
Phosphorus (kg P ha ) CK
0
0
0
0
0
NPK
60
25
58
41
36
NPKS
a
58+4
41
36+8
22+65a
25
58+95a
41
36+66a
hNPKM 39+98a
25
86+143a
−1
Potassium (kg K ha ) CK
0
0
0
0
0
NPK
47
78
69
78
68
NPKS
–
78
68+86a
78
68+92a
NPKM
78 a
9+160
hNPKM 14+240a a
a
69+57
a
78
69+180
78
103+271a –
Estimation of PIC Following Landi et al. (2003), we calculated the amount of PIC as: PIC ¼
δ13 C SIC − δ13 C PM SIC δ13 C PIC − δ13 C PM
ð1Þ
where δ13CSIC, δ13CPM and δ13CPIC were the Stable 13C in carbonate for the bulk SIC, parent material, and pure PIC, respectively. For standard calculation, we set δ13CPM as zero for the Urumqi and Zhengzhou sites, but −1‰ for the Loess at Yangling (according to Liu et al. 2011). Based on Mermut et al. (2000), δ13CPIC was calculated from the Stable 13C in SOC (δ13CSOC): δ13 C PIC ¼ δ13 C SOC þ 14:9
a
25
NPKM
CO2 was determined by a thermal conductivity detector. Total soil carbon was measured using the same procedure without pretreatment of H3PO4. Soil inorganic carbon was calculated as the difference between total soil carbon and SOC. All the procedures were performed at the State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences (CAS). Stable 13C isotopic compositions (δ13C) in SOC and SIC were determined by measuring the isotopic composition of collected CO2 using a Finnigan MAT Delta Plus XP Isotope Ratio Mass Spectrometer at the Nanging Institute of Geology and Paleontology, CAS. For δ13C in SOC, CO2 was collected in the same way as that for SOC. For δ13C in SIC, CO2 was collected during the reaction of pre-heated soil (at 375 °C for 17 h) with H3PO4. We reported isotopic data in delta notation relative to the Vienna Pee Dee Belemnite (VPDB).
ð2Þ
where the value of 14.9 represented an average difference in Stable 13C composition between SOC and PIC, which included the isotopic fractionation of 4.4 for CO2 diffusion and 10.5 for carbonate precipitation (Cerling 1984; Cerling et al. 1989, 1991). Calculations and statistical analyses of SIC and PIC change rates
–
The amount of N/P/K added by crop straw or manure. The manure types were horse manure from 1990 to 1998 and cattle manure from 1999 to 2009 (no application in 2007) at the Zhengzhou site. Cattle and goat manure were applied at Yangling and Urumqi, respectively
For each layer, SIC and PIC stocks were calculated from bulk density and carbon contents. For the Zhengzhou and Urumqi sites where bulk density was only measured for the 0–20 and 20–40 cm layers (Table 1), we used the bulk
Plant Soil
density values from the 20–40 cm layer for calculations of SIC and PIC stocks below 40 cm. Accumulation rates of SIC and PIC were estimated by the increases of the SIC and PIC stocks from 2001/2002 to 2009. We applied oneway analyses of variance (ANOVA) and Fisher’s protected least significant difference (LSD) to evaluate fertilization effects, using Microsoft Excel.
Figure 1 illustrates that SIC content was less than 7 g C kg−1 at the Urmuqi site. There was a decrease in
SIC content with depth, particularly under fertilization treatments that revealed slightly higher SIC in the upper 20 cm layer, but lower SIC below 60 cm. Applying manure caused a modest decrease by 0.4–1.2 g C kg−1 in SIC content in the soils collected in 2001, relative to the NPK treatment. However, there was an increase in SIC over time in association with the long-term application of manure, with a greater increase found below 40 cm (Fig. 1). The δ13CSIC value varied from −1.2 to −4.4‰, with the most negative values found under high rates of mineral and manure application (hNPKM). There was little change in δ13CSIC over time except under the hNPKM treatment. On average, δ13CSIC was −2.4 and −2.8‰ in soils collected in 2001 and 2009, respectively. Similar to SIC, PIC revealed a weak decreasing trend
Fig. 1 Profiles of soil inorganic carbon (SIC, g kg−1: left column), Stable 13C isotopic composition in SIC (‰: middle column), and estimated pedogenic carbonate (PIC, g kg−1: right column) under
non-fertilization (a–c), NPK (d–f), NPKM (g–i) and hNPKM (j–l) for 2001 (white square) and 2009 (white triangle) at the Urumqi site
Results Soil inorganic carbon and isotopic composition at Urumqi
Plant Soil
with depth. Estimated PIC showed little difference in 2001 with a similar range (1–2 g C kg−1) for all the treatments, but a considerable difference in 2009. While there was little increase in PIC over time under nonfertilization and NPK fertilization, there was a significant increase in PIC with manure application. Particularly, the hNPKM treatment resulted in approximately 100 % increase in PIC (Table 3). Soil inorganic carbon and isotopic composition at Yangliang The Yangling site showed a sharp decline in SIC with depth, from 9 to 12 g C kg−1 in the 0–40 cm layer to near zero below 80 cm (Fig. 2), particularly under those without organic matter addition (such as control and NPK). Fertilization led to an increase of 1–2 g C kg−1 in SIC over time, with the greatest increase found under the long-term application of manure. The δ13CSIC value ranged from −5.6 to −8.8‰, showing an approximate value of −6‰ above 60 cm across all the treatments. There was a depletion of 13C in SIC below 60 cm, particularly in the 80–100 cm layer. Estimated PIC showed a similar vertical distribution to that of SIC, with much higher values in the topsoil (6– 8 g C kg−1) than in the 80–100 cm layer (0–2 g C kg−1). Overall, application of organic materials (such as
NPKS, NPKM and hNPKM) resulted in an increase of PIC content over time. Soil inorganic carbon and isotopic composition at Zhengzhou Figure 3 shows a modest decrease in SIC with depth at the Zhengzhou site, from approximately 8 g C kg−1 in the 0–20 cm layer to 4–5 g C kg−1 below 80 cm. Our data showed little difference in the magnitude of SIC between fertilization treatments except under the NPKM treatment that showed higher SIC contents (∼8 g C kg−1) in the 40–60 cm layer. There was an increase in SIC content over time cross all the treatments. A narrow range of the δ13CSIC value (from −4.1 to −5.3‰) was found at the Zhengzhou site. Unlike the Urumqi and Yangliang sites, the δ13CSIC value was a slightly less negative in the subsoil relative to the surface and revealed little difference between 2002 and 2009. Estimated PIC showed a decline from 3 to 4 g C kg−1 in the 0–20 cm layer to approximately 2 g C kg−1 below 80 cm. Fertilization led to an increase of PIC in the whole soil profile over time. Particularly, straw incorporation and manure application in addition to chemical fertilization resulted in increased PIC content by 0.1– 1.2 g C kg−1 from 2002 to 2009.
Table 3 Change rates (g C m−2 y−1) of soil inorganic carbon (SIC) and pedogenic carbonate (PIC) over 0–20 and 20–100 cm under various fertilization treatments Treatment
Urumqi 0–20
Yangling 20–100
0–20
Meana
Zhengzhou 20–100
0–20
20–100
0–20
Totalb 20–100
0–100
SIC Control
−12
29
31
5
12
65
10
33 a
43 (31)
NPK
3
60
31
144
27
39
20
81 ab
101 (64)
137
6
160
2
125
4
142 ab
146 (28)
NPKM
35
85
41
197
30
46
35
109 ab
144 (84)
hNPKM
24
116
16
247
20
182 b
202 (87)
Control
0
5
4
17
6a
14 a
20 (14)
NPK
10
26
25
61
35
22
23 ab
36 ab
60 (25)
97
42
83
14
98
28 ab
91 c
119 (8)
17
57
NPKS
PIC
NPKS NPKM
20
35
50
114
hNPKM
31
136
33
157
13
20
29 ab
69 bc
98 (58)
32 b
147 d
179 (16)
a
Values followed by the same letter in each column are not significantly different, based on a LSD test (P
