Pedosphere 26(2): 257–264, 2016 doi:10.1016/S1002-0160(15)60040-6 ISSN 1002-0160/CN 32-1315/P c 2016 Soil Science Society of China ⃝ Published by Elsevier B.V. and Science Press
Microbial Biomass Dynamics in a Tropical Agroecosystem: Influence of Herbicide and Soil Amendments Alka SINGH, Mahesh Kumar SINGH and Nandita GHOSHAL∗ Centre of Advanced Study in Botany, Department of Botany, Banaras Hindu University, Varanasi 221005 (India) (Received July 2, 2014; revised December 7, 2015)
ABSTRACT The influences of herbicide alone and in combination with the soil amendments with contrasting resource qualities on dynamics of soil microbial biomass C (MBC), N (MBN), and P (MBP) were studied through two annual cycles in rice-wheat-summer fallow crop sequence in a tropical dryland agroecosystem. The experiment included application of herbicide (butachlor) alone or in combination with various soil amendments having equivalent amount of N in the forms of chemical fertilizer, wheat straw, Sesbania aculeata, and farm yard manure (FYM). Soil microbial biomass showed distinct temporal variations in both crop cycles, decreased from vegetative to grain-forming stage, and then increased to maximum at crop maturity stage. Soil MBC was the highest in herbicide + Sesbania aculeata treatment followed by herbicide + FYM, herbicide + wheat straw, herbicide + chemical fertilizer, and herbicide alone treatments in decreasing order during the rice-growing period. During wheat-growing period and summer fallow, soil MBC attained maximum for herbicide + wheat straw treatment whereas herbicide + FYM, herbicide + Sesbania, and herbicide + chemical fertilizer treatments showed similar levels. The overall trend of soil MBN was similar to those of soil MBC and MBP except that soil MBN was higher in herbicide + chemical fertilizer treatment over the herbicide + wheat straw treatment during rice-growing period. In spite of the addition of equivalent amount of N through exogenous soil amendments in combination with the herbicide, soil microbial biomass responded differentially to the treatments. The resource quality of the amendments had more pronounced impact on the dynamics of soil microbial biomass, which may have implications for long-term sustainability of rainfed agroecosystems in dry tropics. Key Words:
chemical fertilizer, farmyard manure, organic amendment, Sesbania aculeata, wheat straw
Citation: Singh A, Singh M K, Ghoshal N. 2016. Microbial biomass dynamics in a tropical agroecosystem: Influence of herbicide and soil amendments. Pedosphere. 26(2): 257–264.
INTRODUCTION Tropical dryland agroecosystems generally have low crop productivity, and the factors responsible are not only limitations of soil moisture and/or nutrients but also the severe weed infestation. Weeds are generally controlled by herbicides (Hyv¨onen and Salonen, 2002). The exogenous soil amendments are also commonly needed for maintaining optimal crop productivity in such nutrient-poor agroecosystems. Herbicides, when added with soil amendments of different resource qualities in the soil may end up with interactive effects. These have important implications for soil microbial biomass dynamics, weed and crop growth, and the long-term soil fertility and crop productivity (Wardle and Rahman, 1992; Moreno et al., 2007). In tropical dryland agroecosystems, addition of organic soil amendments have been recommended over chemical fertilizers as they can improve soil quality, especially soil moisture regime, which is one of the cri∗ Corresponding
author. E-mail: n
[email protected].
tical factors controlling soil productivity. Soil organic inputs, however, vary in their resource qualities, which in turn have major influence on the size of microbial biomass and microbial activity in croplands (Singh et al., 2007; Singh and Ghoshal et al., 2013). Soil microbial biomass, the small and labile fraction of soil organic matter, regulates the availability of major nutrients in soil, and hence is considered widely as an index of soil fertility (Marumoto, 1984; Hassink et al., 1991). An increase in soil microbial biomass is generally linked with improvement in soil fertility. Soil microbial biomass provides a sensitive and early indication of changes in soil quality, as it responds much faster compared to the total soil organic matter in response to changes in agronomic practices (Powlson et al., 1987). Dynamics of soil microbial biomass is reported widely to be influenced by addition of soil amendments (Kandeler et al., 1999; Nayak et al., 2007; Kong et al., 2008) and herbicides alone (Wang et al., 2007; Mah´ıa et al., 2008). However, studies on the interacti-
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on of herbicide and soil amendments with varying resource qualities on soil microbial biomass (Singh and Ghoshal, 2010) are limited in general, and the rainfed dryland agroecosystem in particular. The main objective of this study, carried out in a tropical dryland agroecosystem with rice-wheat-summer fallow crop sequence, was to evaluate the impact of application of herbicide alone and in combination with soil amendments having contrasting chemical qualities on the dynamics of soil microbial biomass C (MBC), N (MBN), and P (MBP).
wing once a year. Fresh Sesbania aculeata shoots were cut into pieces (2–3 cm) before incorporation. Wheat straw was air-dried, and then incorporated. Chemical fertilizer was surface applied. Herbicide was applied 1 or 2 d after sowing of rice crop. No exogenous inputs were applied to the wheat. Manual hoeing was thoroughly done up to 15-cm depth to prepare the plots for the sowing of rice or wheat. Both the crops were directly seeded in the soil. The experimental setup was maintained continuously since its establishment in June 2004.
MATERIALS AND METHODS
Soil sampling and analysis
Study site This study involved two annual cycles (2008–2010) in the experimental plots of the Department of Botany, Banaras Hindu University at Varanasi (25◦ 18′ N, 83◦ 1′ E, and 76 m above sea level) characterized by three different seasons, i.e. summer, rainy, and winter, which is typical of the dry tropical region. The soil belongs to the order Inceptisol, suborder Orchrepts of the subgroup Udic Ustocrepts, with pale brown coloration and sandy loam texture. With a pH of 6.7, the soil had a bulk density of 1.36 g cm−3 , water holding capacity of 39.0%, organic C of 6 g kg−1 , total N of 0.58 g kg−1 , and total P of 92 mg kg−1 . The experimental design included application of herbicide alone or in combination with various soil amendments having equivalent amount of N (i.e., 80 kg N ha−1 ). Butachlor was used as herbicide at the dose of 2 kg a.i. ha−1 . Three organic amendments, wheat straw, Sesbania aculeata, and farm yard manure (FYM), as well as an inorganic amendment, chemical fertilizer, were used. The experiment consisted of six treatments including a control (with no inputs), herbicide alone (HA), herbicide + chemical fertilizer (HC, chemical fertilizer application at 80-40-30 kg ha−1 NP-K as urea, single super phosphate, and muriate of potash), herbicide + wheat straw (HW, C:N = 82:1), herbicide + Sesbania aculeata (HS, C:N = 16:1), and herbicide + farmyard manure (HF; C:N = 28:1). Periods for rice (Oryza sativa var. NDR 97) growing were from July to October in 2008 and 2009, respectively. Wheat (Triticum aestivum var. Malviya 533) cropping extended from November in 2008 to March in 2009 and November in 2009 to March in 2010. Summer fallows ranged from April to June in 2009 and 2010, respectively. The experimental plots (4 m × 4 m) were laid down in a randomized block design with three replicates, with a 1-m strip separated from each block. The inputs were applied 1 or 2 d before rice so-
Soil samples were collected from each treatment at three growing stages of rice and wheat (vegetative, grain-forming, and maturity stage) and summer fallow for each annual cycle, amounting to 14 samples during the two-year study. In each replicate plot, soil samples were randomly collected from 0–10 cm depth from three spots, mixed, and then sieved through a mesh screen (2 mm), following removal of the visible plant debris. Field moist soil samples were preconditioned for 7 d at room temperature in a container with 100% humidity and CO2 removed by alkali contained in a vial. Modified chloroform fumigation-extraction method was used for estimation of MBC (Vance et al., 1987), MBN (Brookes et al., 1985), and MBP (Brookes et al., 1982). Statistical analyses Data were analysed using SPSS package. All the values were expressed as means ± standard errors. Means were compared using the least significant difference (LSD) test. Differences between treatments and crop cycles were tested using two-way analysis of variance (ANOVA). Significance of difference was indicated at P < 0.05. RESULTS Application of herbicide alone or in combination with soil amendments showed a trend of increase in the levels of soil microbial biomass compared to the control treatment throughout the annual cycle (Figs. 1– 4). In all the treatments the levels of soil microbial biomass increased from rice- to wheat-growing period and reached the maximum level in summer fallow except in HS treatment. Temporal variations were distinct and similar in soil MBC, MBN, and MBP across all the treatments for rice and wheat growing periods. In both crop cycles, the microbial biomass decreased from vegetative to grain-forming stage, and then increased until maturity stage.
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Fig. 1 Variations in soil microbial biomass C due to application of herbicide alone or in combination with various soil amendments during the three growing stages (vegetative, grain-forming, and maturing stages) of rice (R) and wheat (W) and summer fallow (S) for the two annual cycles. Vertical bars indicate standard errors of the means (n = 3). HA = herbicide alone; HC = herbicide + chemical fertilizer; HW = herbicide + wheat straw; HF = herbicide + farmyard manure; HS = herbicide + Sesbania aculeata.
During rice-growing period, the highest level of soil MBC was found in HS treatment followed, in decreasing order, by HF, HW, HC, H, and control treatment (Figs. 1 and 4). During wheat-growing period, soil MBC increased considerably from rice-growing period to wheat-growing period in HW treatment, resulting in the highest level among all the cultivated plots, whereas similar levels of soil MBC were found in HF, HS, and HC treatments. The trend of soil MBC in each treatment during wheat-growing period was: HW > HF > HS > HC > HA > control, which was almost the same as that observed during summer fallow (Figs. 1 and 4). The effects of herbicide and amendment application on soil MBP (Figs. 2 and 4) were in line with the soil MBC (Figs. 1 and 4) across all the treatments throughout the annual cycle. During rice-growing pe-
riod, the soil MBP was the highest in HS treatment, whereas it was highest in HW treatment during wheatgrowing period. Similar trend in various treatments was found during summer fallow. The overall trend of soil MBN was similar to that of soil MBC and MBP, except that the MBN level was higher in HC treatment than HW treatment during rice-growing period (Figs. 3 and 4). During this period, soil MBN was maximum found in HS treatment followed in decreasing order by HF, HC, HW, and HA treatment. However, during wheat-growing period, HW treatment yielded the maximum soil MBN followed in decreasing order by HF, HS, HC, and HA treatment. The trend of MBN during summer fallow was entirely similar to that during the wheat-growing period across treatments. In terms of mean of crop cycles, soil MBC, MBN,
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Fig. 2 Variations in soil microbial biomass P due to application of herbicide alone or in combination with various soil amendments during the three growing stages (vegetative, grain-forming, and maturity stages) of rice (R) and wheat (W) and summer fallow (S) for the two annual cycles. Vertical bars indicate standard errors of the means (n = 3). HA = herbicide alone; HC = herbicide + chemical fertilizer; HW = herbicide + wheat straw; HF = herbicide + farmyard manure; HS = herbicide + Sesbania aculeata.
and MBP showed the increasing trend with the application of four other amendments, and those in HA treatment were comparable with the control treatment (Table I). Significantly higher soil MBC, MBN, and MBP were observed in treatments with organic amendments (HS, HF, and HW) than the treatment amended with chemical fertilizer (HC). Two-way ANOVA of the data for two annual cycles showed that soil MBC, MBN, and MBP were more influenced by treatments rather than the variations caused by crop cycles or their interactions (Table II). DISCUSSION Impacts of soil amendments on soil microbial biomass Microbial biomass responded differentially to herbicide addition combined with soil amendments of
varying resource qualities. The utilization of the readily mineralizable nutrient from rapidly decomposing Sesbania aculeata with lower C:N ratio might have favoured higher MBC, MBN, and MBP during rice-growing period. The utilization of nutrients by microbes or their uptake by plants might have lowered their availability during wheat-growing period as reflected by the considerably lowered soil microbial biomass during the later phase of annual cycle (Figs. 1– 4). Higher microbial biomass during first crop (rice)growing period and the marginal effect on the second crop (wheat)-growing period due to single application of Sesbania were reported earlier (Yadvinder-Singh et al., 2004; Singh et al., 2007). In the current study, the pattern of microbial biomass was similar to that of single application of Sesbania rather than single application of herbicide, properly because the impact of herbi-
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Fig. 3 Variations in soil microbial biomass N due to application of herbicide alone or in combination with various soil amendments during the three growing stages (vegetative, grain-forming, and maturity stages) of rice (R) and wheat (W) and summer fallow (S) for the two annual cycles. Vertical bars indicate standard errors of the means (n = 3). HA = herbicide alone; HC = herbicide + chemical fertilizer; HW = herbicide + wheat straw; HF = herbicide + farmyard manure; HS = herbicide + Sesbania aculeata. TABLE I Variations in soil microbial biomass C (MBC), N (MBN), and P (MBP) during two annual cycles under different treatments in a rice-wheat-summer fallow dryland agroecosystem Treatmenta)
2008–2009 annual cycle MBC
Control HA HC HW HF HS
151 167 212 242 251 256
LSD valuec)
24.4
a) HA
± ± ± ± ± ±
2009–2010 annual cycle
MBN 8b) 8 9 11 8 7
18.2 20.8 24.7 29.9 30.6 31.5 2.6
± ± ± ± ± ±
MBP 0.7 0.7 0.7 1.4 0.9 1.0
7.2 8.0 9.4 14.8 14.5 13.1 1.05
± ± ± ± ± ±
MBC µg 0.2 0.2 0.3 0.7 0.3 0.3
g−1
dry soil 145 165 207 237 242 247 22.9
± ± ± ± ± ±
MBN 7 8 8 10 8 8
17.3 19.3 23.7 28.6 29.0 29.9 2.5
± ± ± ± ± ±
MBP 0.8 0.7 0.7 1.2 0.8 1.0
6.9 7.7 9.1 14.5 14.1 12.7
± ± ± ± ± ±
0.2 0.2 0.3 0.9 0.3 0.4
1.07
= herbicide alone; HC = herbicide + chemical fertilizer; HW = herbicide + wheat straw; HF = herbicide + farmyard manure; HS = herbicide + Sesbania aculeata. b) Means ± standard errors. c) Least significant difference (P < 0.05).
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Fig. 4 Variations in soil microbial biomass C, P, and N due to application of herbicide alone or in combination with various soil amendments during the growing periods of rice (R) and wheat (W), and summer fallow (S) for the two annual cycles. Values for riceand wheat-growing period are the means of vegetative, grain-forming, and maturity stages during each annual cycle. Vertical bars indicate standard errors of the means (n = 3). Bars with the same letter(s) are not significantly different at P < 0.05. HA = herbicide alone; HC = herbicide + chemical fertilizer; HW = herbicide + wheat straw; HF = herbicide + farmyard manure; HS = herbicide + Sesbania aculeata. TABLE II Analysis of variance for microbial biomass C (MBC), N (MBN), and P (MBP), using treatment and crop cycle as factors in 2008– 2010 Factor
dfa) F value MBC
Treatment 5 Crop cycle 13 Treatment × crop cycle 65
MBN
MBP
281*** 150*** 1854*** 69*** 13*** 108*** 2.78*** 2.51*** 19.52***
***Significant at P < 0.001. a) Degree of freedom.
cide addition was masked by the application of Sesbania aculeata. Accumulation of higher soil MBC, MBN, and MBP throughout the annual cycle in HF treatment
was probably due to sustained availability of nutrients through the decomposition of different fractions of FYM (Sluijsmans and Kolenbrander, 1977), which seems not to be interfered by herbicide application. The presence of high biogenic materials in FYM can enhance the microbial biomass production in amended soils (Bhattacharyya et al., 2005). Higher microbial biomass due to FYM addition was also reported (Kandeler et al., 1999; Marschner et al., 2003). Lower soil MBC, MBN, and MBP during ricegrowing period and high during later wheat-growing period of annual cycle in HW treatment in the current study were probably due to higher C:N ratio of wheat straw. Higher C:N ratio resulted in early immobilization of nutrients due to slow and delayed decomposition, which remineralized and released later in the an-
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nual cycle (Yadvinder-Singh et al., 2004). The availability of nutrients during the second crop-growing period rather than the first one was also reported earlier (Aulakh et al., 2000). The accumulation pattern of soil microbial biomass was influenced by the single application of wheat straw rather than the herbicide application (Singh and Ghoshal, 2010). The soils of this region are not only nutrient- and moisture-limited but also poor in organic C. The application of fertilizers, a rich and ready source of available nutrients, may overcome the nutrient deficiency, even if the C requirement was not satisfied, and this might be the reason for the lowest microbial biomass in HC treatment. Similar to other three organic amendment treatments, the impact of chemical fertilizer addition was more pronounced over herbicide. Higher microbial biomass as a response to additions of organic inputs compared to chemical fertilizer was reported by B¨ohme and B¨ohme (2006), Bhattacharyya et al. (2005), and Mandal et al. (2007). However, high soil MBC was reported in fertilizer treatment compared to compost (Nayak et al., 2007). Herbicide application at the recommended dose in this study did not result any significant change in soil MBC, MBN, and MBP compared to the control treatment, probably because the effects of herbicide application on microbial biomass is short-term and relatively insignificant compared with the seasonal variations (Subhani et al., 2000). Hart and Brookes (1996) reported no effect on microbial biomass after 19 years of annual field applications of pesticides (glyphosate, benomyl, chlorfenvinphos, and triadimefon) applied at the recommended rates. Singh and Ghoshal (2010) reported reductions in the microbial biomass due to the application of herbicide, whereas Moreno et al. (2007) reported higher levels of microbial biomass.
have resulted from the competition for nutrients among crop roots and microbes (Ghoshal and Singh, 1995). The competition for nutrients was maximum during grain-forming stage and minimum after harvest during summer fallow (Ghoshal and Singh, 1995). Singh et al. (2007) showed a strong negative correlation between microbial biomass and crop root biomass through the crop cycles. Similar temporal trend of microbial biomass was also reported by Bhattacharyya et al. (2005) and Ghoshal and Singh (2010). However, contrasting trends showing increase in the size of the microbial biomass with the crop growth (Franzluebbers et al., 1995) or no significant change during the year (Patra et al., 1990) were also reported.
Temporal variations of soil microbial biomass Although the ANOVA showed that the effect of soil amendments on soil microbial biomass were more pronounced than their temporal variations yet, distinct temporal variation was found in this study (Table II). The temporal patterns of soil MBC, MBN, and MBP among all the treatments were similar in both the crop cycles in spite of the fact that rice and wheat were grown in different climatic regimes (Figs. 1–4). Debosz et al. (1999) reported that the major regulators of temporal variations are climatic factors and crop growth, but not the amount of annual organic inputs. The considerably lowering of microbial biomass during grain-forming stage from the vegetative stage, and subsequent increase during crop maturity stage might
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