Soil aggregate and microbial biomass in a permanent bed wheat–maize planting system after 12 years
Introduction
Permanent-bed planting system as a form of conservation tillage has been proposed for wheat and maize production in the irrigated area of northwest Mexico in the Yaqui Valley (Limon-Ortega et al., 2000). This system consists of planting two seed-rows of wheat on the top of raised-beds, and retaining plant residues from the previous crop, which is chopped and left in the field as stubble.
Planting on conventional-tilled beds increases water use efficiency and decreases operational costs compared with conventional tilled wheat without beds (Aquino, 1998, Fahong et al., 2004). Bed planting provides a natural opportunity to reduce soil compaction by confining traffic to the furrows. A next, logical step to increase their sustainability is to reduce tillage and manage plant residues from the previous crop on the surface, and reshape the beds only as needed between crop cycles.
Crop production practices such as tillage, plant residue management, N fertilization and crop rotation influence a number of biological and physical soil attributes. Reducing tillage combined with crop residue retention on the soil surface as stubble can increase water infiltration (Bruce et al., 1990, Carter, 1992, Azooz and Arshad, 1996, Elliot and Efetha, 1999, Shaver et al., 2002), reduce soil erosion, increase water use efficiency (Johnston et al., 2002, McGarry, 2002) and reduce water loss by evaporation. Crop rotations, on the other hand, can break soil borne pathogen cycles and reduce weed pressure (Karlen et al., 1994).
The soil microbial biomass C and N (SMB C and N, respectively) reflect the soil's ability to store and cycle nutrients (C, N, P and S) and organic matter, and has a high turnover rate relative to the total soil organic matter (Dick, 1992, Carter et al., 1999). Due to its dynamic character, microbial biomass responds to changes in soil management often before effects are measured in terms of organic C and N (Powlson and Jenkinson, 1981, Carter and Rennie, 1982). In agricultural soils, 200–1000 μg biomass C g−1 soil is often found, which can fix 100–600 kg N ha−1 in the upper 30 cm of soil (Martens, 1995). However, it is uncertain whether the microbial biomass provides potentially mineralizable N through its reduction in size or by mineralizing labile N from the organic fraction during a growing season (Dalal, 1998).
Soil microbial biomass provides key information about the capacity of the soil to function (Seybold and Herrick, 2001) and plays an important role in physical stabilization of aggregates (Franzluebbers et al., 1999, Doran et al., 1998). A common method to assess aggregate stability is through wet sieving procedures (Yoder, 1936), which can be parameterized by means of fractal dimension (D). This approach has been applied to dry sieving procedures and indicates in both cases that soils with a substantial proportion of stable and large aggregates will show low D values, while less stable and more fragmented soils, high D values (Perfect and Kay, 1991) from wet and dry sieving, respectively. The information provided by those two parameters from wet and dry sieving can be extended to calculate the soil dispersion index (ILACO, 1985) through the numerical difference between D's. This index adjoins simultaneous information on stability and size of aggregates to compare differences among treatments, and is denoted in this document as delta D (ΔD).
Microbial biomass has been reported as responsible for soil aggregation (Gupta and Germida, 1988), but relationships between SMB and aggregate stability are not very consistent and tend to be mainly site-specific (Carter et al., 1999). For example, Angers et al. (1992) reported linear correlation coefficients between microbial biomass C and water-stable aggregates that varied from 0.40 to 0.60. Although this relationship could be slightly curvilinear, the statistical approach generally used to obtain these coefficients is considered satisfactory (Chaney and Swift, 1984).
The objective of this study was to compare soil attributes in a long-term experiment initiated in northwest Mexico in 1992. The trial includes conventional-till and permanent bed treatments with differential plant residue management and N rates. Measurements were taken in the 2002 and 2004 wheat crop seasons and include the soil microbial biomass C and N and soil structure parameterized by means of fractal dimension from both dry and wet sieving.
Section snippets
Materials and methods
In 1992, the experiment was initiated at the CIANO (Centro de Investigaciones Agrícolas del Noroeste) research station near Ciudad Obregón in Sonora, Mexico (latitude 27.33°N, longitude 109.09°W, 38 m above sea level). The soil type at CIANO station is a coarse sandy clay, mixed montmorillonitic Typic Calciorthid, low in organic matter (<1%) and slightly alkaline (pH 7.7). A description of the plot management has already been reported in Limon-Ortega et al. (2000).
The experiment included three
Results
Soil aggregation from dry sieving varied significantly with tillage-plant residue treatments (Table 1), while N fertilizer treatments had no effect (data not shown). Incorporation of plant residues in CTB treatments showed the highest degree of aggregation (low D value) and PB-all residues burned the lowest (large D value). According to Fig. 1.
Although the CTB-all residues incorporated showed a high degree of aggregation from dry sieving (D), the large dispersion index for this treatment (ΔD =
Discussion
Although other reports have indicated that heat from burning barely penetrates the soil to below 1cm (Biederbeck et al., 1980), results from this study for grain yield, soil fractionation, SMB C and N, indicate that the practice of burning should be discouraged, as it influences soil quality to at least 7.5 cm depth. The apparent stability of aggregates from PB-all residues burned is attributed to highly fractionated aggregates that resist further break down by wet sieving. Those highly
Conclusions
Results in this work indicate that the long-term use of PB-all residues retained as stubble system from wheat and maize in rotation improves soil aggregation and stability and increases C and N from the SMB over time. Even though the N improvement from the SMB was associated with a reduced kernel weight, plants in PB-all residues retained as stubble showed an enhanced tillering ability, which increased the number of spikes/m2, which compensated for the reduced kernel weight, providing higher
Acknowledgement
B.G. received grant-aided support from the Flemish Interuniversity Council (VLIR-UOS), Belgium.
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