Variations in soil microbial biomass and N ... - Semantic Scholar

Soil & Tillage Research 56 (2000) 153±166

Variations in soil microbial biomass and N availability due to residue and tillage management in a dryland rice agroecosystem C.P. Kushwaha, S.K. Tripathi, K.P. Singh* Department of Botany, Banaras Hindu University, Varanasi 221005, India Received 18 January 2000; received in revised form 30 May 2000; accepted 8 June 2000

Abstract Seasonal changes in the levels of soil microbial biomass C (MBC) and N (MBN), N-mineralization rate and available-N concentration were studied in rice±barley supporting tropical dryland (rainfed) agroecosystem under six combinations of tillage (conventional, minimum and zero tillage) and crop residue manipulation (retained or removed) conditions. Highest levels of soil MBC and MBN (368±503 and 38.2±59.7 mg gÿ1, respectively) were obtained in minimum tillage residue retained (MTR) treatment and lowest levels (214±264 and 20.3±27.1 mg gÿ1, respectively) in conventional tillage residue removed (CTÿR, control) treatment. Along with residue retention tillage reduction from conventional to zero increased the levels of MBC and MBN (36±82 and 29±104% over control, respectively). The proportion of MBC and MBN in soil organic C and total N contents increased signi®cantly in all treatments compared to control. This increase (28% in case of C and 33% N) was maximum in MTR and minimum (10% for C and N both) in minimum tillage residue removed (MTÿR) treatment. In all treatments concentrations of N in microbial biomass were greater at seedling stage, thereafter these concentrations decreased drastically (21±38%) at grain-forming stage of both crops. In residue removed treatments, N-mineralization rates were maximum during the seedling stage of crops and then decreased through the crop maturity. In residue retained treatments, however, N-mineralization rates were lower than in residue removed treatments at seedling stage of both crops. At grain-forming stage in all instances the N-mineralization rates in residue retained treatments considerably exceeded the rates in corresponding residue removed treatments. Tillage reduction and residue retention both increased the proportion of organic C and total N present in soil organic matter as microbial biomass. Microbial immobilization of available-N during the early phase of crops and its pulsed release later during the period of greater N demand of crops enhanced the degree of synchronization between crop demand and N supply. The maximum enhancement effects were recorded in the minimum tillage along with residue retained treatment. In the dryland agroecosystem studied, two management practices in combination proved more advantageous than either practice alone in maintaining soil fertility levels. For soil fertility amelioration in dryland agroecosystems with least dependence upon chemical fertilizer input, post-harvest retention of about 20 cm shoot biomass (accounting for 25±40% aboveground biomass) of previous crop and its incorporation in soil through minimum tillage in the succeeding crop is suggested, especially in the case of cereal. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Dryland agriculture; Tillage reduction; Microbial biomass; N-mineralization rate; Available N; Residue retention

* Corresponding author. Tel.: 91-542-317099; fax: 91-542-317074. E-mail address: [email protected] (K.P. Singh).

0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 1 9 8 7 ( 0 0 ) 0 0 1 3 5 - 5

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C.P. Kushwaha et al. / Soil & Tillage Research 56 (2000) 153±166

1. Introduction Agroecosystems in the tropics are faced with the biological degradation of soils, which results in reduction in organic matter content due to decline in the amount of carbon input from biomass (Stewart and Robinson, 1997). In dryland (rainfed) agroecosystems excessive tillage and crop residue removal leads to water scarcity and low soil fertility, both responsible for decreased crop production (Singh and Singh, 1995). In dryland agriculture use of conservational tillage and organic inputs have been suggested as a method of `low-input agriculture' to achieve sustainable crop production (Singh and Singh, 1993). Interest in the conservational tillage, an ecological approach to seed-bed preparation, has increased recently (from 15 to 38 million hectare over the last decade, CTIC, 1994) because of the potential of this practice to reduce soil erosion, conserve soil moisture and improve soil fertility. According to Blevins and Frye (1993) conservational tillage practices are responsible for temporary immobilization of fertilizer N and reduction in N availability to crops at early stages of growth. Thus, there is a need to understand the impact of practices combining tillage and residue input variations on the biological processes involved in the maintenance of soil fertility in relation to the sustainable crop production systems. Microbial biomass, a small (1±5% by weight) but active fraction of soil organic matter is of particular concern in soil fertility considerations because it is more susceptible to management practices than the bulk organic matter (Janzen, 1987). Soil microbial biomass acts as a reservoir of critical nutrients (Smith and Paul, 1990) and is a major determinant for governing the nutrient availability and resource base for nutrient release, which ®nally re¯ect soil fertility levels. Microbes using the organic materials as an energy source compete with crop plants for available N. Microbial biomass temporarily accumulates available N and gradually releases it into soil with passage of time. Presence of low quality crop residues in association to fertilizer application will extend the time period of availability of N to the crop plants in dryland farming conditions through the initial immobilization of N in the microbial biomass. Addition of plant residues with high C:N ratio may facilitate transformation of fertilizer or soil N into a slowly

available N source and thus may improve N use ef®ciency in dryland agroecosystems (Singh and Singh, 1995). Microbial biomass under upland ®eld conditions may practically act as slow release fertilizer. Last two decades have seen a great interest in the development of farming systems characterized by relatively inexpensive levels of inputs combined with a high ef®ciency of internal resource recycling (Woomer and Ingram, 1990). Organic materials like crop residues offer sustainable and ecologically sound alternatives for meeting the N requirement of crops. The suitability of crop residues as a source of N depends to a great extent on the mineralization of its N in relation to the crop demand. The decomposition of organic matter in soil and the accompanying mineralization and immobilization of inorganic N are key processes in the soil±plant N cycle (Watkins and Barraclough, 1996). Crop productivity is strongly in¯uenced by nutrient availability in soil and the nutrient supply rate (N-mineralization) is a crucial process of nutrient dynamics (Binkely and Vitousek, 1989). The largest proportion of N is found in the soil organic matter and its availability to plant is dependent on its mineralization to ammonium-N which under favourable conditions may be nitri®ed to nitrate-N. However, the impact of conservational tillage and crop residue retention after harvest on the dynamics of microbial biomass, N-mineralization rate and available nutrients are poorly documented in tropical dryland agroecosystems. Such information may be helpful in developing sustainable crop production practices in dryland agroecosystems in India, which account for ca. 70% of the cultivated land in the country. Farmers in these regions require soil working and organic input management ecotechnology which gives reasonable increase in crop productivity within a short term. This study addresses the following questions with respect to tropical rice±barley dryland agroecosystem when subjected to short term crop residue and tillage manipulation: (1) What is the impact of tillage and residue manipulation on the levels of microbial biomass C and N? (2) Whether the change in N-mineralization rate is associated with the changes in the levels of microbial biomass C and N under conditions of tillage and residue manipulation? (3) How does the amount of available-N change under tillage and


residue manipulation? (4) How does the tillage and residue manipulation affect the N-mineralization? 2. Materials and methods 2.1. Study site This study was performed at the Dryland Farm at the Institute of Agricultural Sciences, Banaras Hindu University (258180 N latitude and 83810 E longitude, 76 m above the mean sea level). The region has a tropical moist sub-humid climate, characterized by strong seasonality with respect to temperature and precipitation. The year is divisible into a warm-wet rainy season (July±September), a cool-dry winter (November±February), and a hot-dry summer (April±June). October and March constitute transitional months between rainy and winter, and between winter and summer seasons, respectively. The summer is dry and hot with temperatures ranging between 35 and 458C during the day. Warm conditions (25±358C) and high relative humidity (70±91%) prevail during the rainy season. In the winter season temperature falls between 10 and 258C. Of the total annual rainfall (ca.

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1287 mm) more than 85% occurred within the rainy season (Fig. 1). The rainy and winter seasons are the major cropping seasons in this region where rice based crop rotation is most common. Rice (Oryza sativa var. NDR 118) was selected as the rainy season test crop and barley (Hordeum vulgare L. var. Joyti) as the winter season crop. The soil of the Banaras Hindu University campus is characterized as Banaras Type III by Agarwal and Mehrotra (1952). It is an inceptisol with a ¯at topography, pale brown colour, and sandy loam texture. 2.2. Experimental design The experiment was designed to vary the amount of organic matter input through residue retention from previous crop to the succeeding crop, and the soil disturbance in the form of different tillage practices (viz. conventional tillage, minimum tillage and zero tillage). Rice was grown in a conventionally tilled ®eld (65 m45 m) during the rainy season in 1997. When the rice crop matured (November 1997) six treatments which replicated thrice (plot size 9 m10 m) were established in a randomized block design. A strip of 1 m was left to separate treatment plots from each

Fig. 1. Ombrothermic diagram for the study area. Solid circles represent rainfall and open circles show mean monthly temperature.

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other. The assessment of initial soil fertility status (November 1997) showed that differences in organic C and total N contents between different replication blocks were not signi®cant; the means (S.E.) for the three blocks were: organic C 785580 mg gÿ1, total N 8728 mg gÿ1. The six treatments were: (1) conventional tillage (disked twice, cultivated once to 20 cm depth), and residue removed (CTÿR); (2) conventional tillage (as above), and residue retained (CTR); (3) minimum tillage (disked once, cultivated once up to 10 cm depth), and residue removed (MTÿR); (4) minimum tillage (as above), and residue retained (MTR); (5) zero tillage (no cultivation other than the disturbance caused by the planting tines), and residue removed (ZTÿR); (6) zero tillage (as above), and residue retained (ZTR). In the residue removed treatments harvesting of rice was done at the ground level, leaving no standing aboveground residue. However, in the residue retained treatments, rice was harvested 20 cm above the ground level leaving a portion of the rice aboveground residue standing in the plot. Rice residue retained in different plots was incorporated in the soil during tillage operations (either conventional or minimum) done before the sowing of barley (October 1997). In the case of zero tillage the rice residue standing in the plot was chopped at ground level and the cut material was left on the soil surface at the time when ®rst tillage operation was being done in conventional and minimum tillage plots. When barley matured (April 1998) its harvesting was also done as in the preceding rice crop and the same six treatments with barley shoot residue were established for the succeeding rice crop.

The chemical composition and the amount of rice and barley residues retained in various treatments are given in Table 1. The quantity of residue retained corresponded to 35±40% of maturity aboveground biomass in rice and 42% (CTR) to 52% biomass (ZTR) in barley. The NPK chemical fertilizers (80 kg N haÿ1, 40 kg P haÿ1 and 30 kg K haÿ1 for rice crop and 60 kg N haÿ1, 40 kg P haÿ1 and 30 kg K haÿ1 for barley crop) were applied in all treatments at the time of crop sowing. 2.3. Soil collection For the estimation of microbial biomass C and N, available-N and N-mineralization, soil samples (0± 10 cm depth) were collected six times, once during seedling, grain-forming, and maturity stages of barley as well as rice crops. Three sub-samples of soil were collected from each replicate plot and composited. After removal of visible plant debris and fauna, soil was sieved through a 2 mm mesh screen. For physicochemical analysis soil was collected after the harvest of rice (November 1998). Samples from different replicate plots of each treatment were analysed separately. Field moist samples were used for the determination of soil pH. 2.4. Soil physico-chemical analysis Soil pH was determined by a pH meter using a glass electrode (1:5, soil:water) and water holding capacity by perforated circular brass boxes (Piper, 1966). Bulk density was determined using a metallic tube of known internal volume. Total organic carbon in soil

Table 1 Chemical composition of crop residues retained and their quantity in different treatments Chemical composition/amount

a

Carbon (%) Nitrogen (%)b C:N ratio Quantity of residue (kg haÿ1)

a

Crop residue Rice

Barley

49.10.52 0.6020.025 81.6 6825358 (in all residue retained treatments)

48.90.53 0.6220.020 78.6 5831495 (in CTR)c 6687308 (in MTR)c 3750508 (in ZTR)c

Determined as: loss in ignition/2 (McBrayer and Cromack, 1980). Micro-kjeldahl method (Jackson, 1958). c Differences in the quantity of residue retained in various treatments were due to the variations in standing crop biomass. b


was analysed following dichromate oxidation and back titration of unused dichromate (Kalembasa and Jenkinson, 1973). Total nitrogen was measured by the microkjeldahl method (Jackson, 1958). 2.5. Estimation of soil microbial biomass MBC was measured by determining the organic carbon in chloroform fumigated and non-fumigated samples by dichromate digestion as described by Vance et al. (1987). Soil MBC was then estimated from the equation: MBC2.64EC (Vance et al., 1987), where EC is the difference between carbon extracted from the K2SO4 extract of fumigated and non-fumigated soils, both expressed as mg C gÿ1 oven dry soil. With the same K2SO4 soil extract, MBN was determined as total nitrogen using kjeldahl digestion procedure (Brookes et al., 1985). The ¯ush of total N (K2SO4 extractable N from non-fumigated soil subtracted from that of fumigated soil) was divided by KN (fraction of biomass N extracted after chloroform fumigation) value of 0.54 (Brookes et al., 1985). 2.6. Determination of available-N and Nmineralization rate Fresh ®eld moist and sieved (2 mm) samples were used for the determination of nitrate-N and ammonium-N concentrations. Nitrate-N was measured by the phenol disulphonic acid method, using CaSO4 as the extractant (Jackson, 1958). Ammonium-N was extracted by 2 M KCl and analysed by the phenate method (American Public Health Association, 1995). N-mineralization rate was measured by buried bag technique (Eno, 1960). Two sub-samples (ca. 150 g each) enclosed in sealed polyethylene bags were buried within 0±10 cm soil depth in each replication plot. Coarse roots and any large fragments of organic debris were removed in order to avoid any marked immobilization during incubation. At time zero (immediately after collection) and after 30 days of ®eld incubation nitrate-N and ammonium-N were determined (as previously described). The increase in the concentration of ammonium-N plus nitrate-N after the ®eld incubation was de®ned as net N-mineralization, the increase in ammonium-N as ammoni®cation, and the increase in nitrate-N as nitri®cation. All the results are expressed on oven dry soil (1058C) basis.

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2.7. Statistical analysis Statistical analysis was done using SPSS/PC software on microcomputer. Data were analysed through analysis of variance (ANOVA) and by correlation and regression to test treatment effects. Treatment means were compared using the LSD range test procedure at the 5% level of signi®cance. 3. Results and discussion 3.1. Soil physico-chemical properties Within a short term after one annual cycle water holding capacity, pH, and bulk density were affected by residue retention and tillage reduction but the effect of residue retention was greater than the effect of tillage reduction except in case of bulk density where the role of tillage reduction was prime (Table 2). Tillage reduction (MTÿR, ZTÿR) did not signi®cantly affect the organic C and total N levels in the soil (Table 2) which is poor in these respects. In contrast, in no-tillage systems Arshad et al. (1990) showed quantitative as well qualitative improvements in soil organic matter and reported about 26% higher organic C and total N in no-tilled soil than in conventionally tilled soil. An increase of 12±25% in organic C was reported under zero tillage compared to conventional tillage (Dick, 1983). In a study comparing 13 years of conventional tillage to zero tillage on a clay loam in southeastern United States, Beare et al. (1994) found 18% increase in soil organic C in zero tillage relative to conventional tillage. On the other hand, in all tillage variations in the present study residue retention tended to increase organic C and total N levels (cf. residue removed treatments). Such increase was more marked in CTR (26% organic C and 36% total N) and MTR (42% organic C and 53% total N). Residue retention in association with tillage reduction (MTR, ZTR) signi®cantly increased soil organic C and total N contents over control. Much less increase was recorded in ZTR (10% organic C, and 12% total N) than in MTR treatment (42% organic C and 53% total N). Saf®gna et al. (1989), however, in subtropical Australian vertisol showed 15±18% increase in soil organic C and total N due to combined effect of

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Table 2 Changes in soil properties in various treatments after one annual cycle of crop growth (barley followed by rice); the values are meanS.E.; treatment code: CTÿR, conventional tillage and residue removed; CTR, conventional tillage and residue retained; MTÿR, minimum tillage and residue removed; MTR, minimum tillage and residue retained; ZTÿR, zero tillage and residue removed; ZTR, zero tillage and residue retained Treatments

Soil properties pH

Bulk density (g cmÿ3)

WHC (g kgÿ1)

Organic carbon (mg gÿ1)

Total nitrogen (mg gÿ1)

C/N ratio

CTÿR CTR MTÿR MTR ZTÿR ZTR

6.850.018 6.760.014 6.840.016 6.750.016 6.750.007 6.720.008

1.270.006 1.240.007 1.290.007 1.250.006 1.420.004 1.400.004

4122.6 4231.9 4163.2 4310.9 4100.6 4142.4

7800208 9800231 8200153 11100173 8100154 8600152

87017 118013 9156 133017 8895 97719

8.96 8.30 8.95 8.34 9.10 8.80

LSD (p