Decomposition of Crop Residues
Voroney, Robert Paul
Crop residues are the prime source of energy and nutrients affecting soil biological, chemical and physical processes, including organic matter formation. This study examined the effects of management on the rate of decomposition of crop residues in the laboratory and field. It also measured the incorporation of crop residue and soil C and N into the microbial biomass. 14C-, 15N-labelled maize residues incorporated and incubated in an aerobic environment, under laboratory conditions, were mineralized to a greater extent in the short term than the surface-placed residues. Residue C, incubated under waterlogged conditions, mineralized more slowly -compared to residue C under an aerobic incubation. Initially net mineralization of residue N was more rapid in the waterlogged treatment. In the long term, the net mineralization results were similar to those where residues were incorporated and incubated aerobically. Changes in microbial biomass during maize residue decomposition illustrated its capacity to act as a major source-sink of energy and nutrients as well as a biocatalyst in the transformation of organic matter. Maximum production of microbial biomass occurred within 8-14 days and accounted for 24% of the C and 34% of the N added. The data provided evidence for the relatively high yield coefficient of C (>40%) in formation of microbial biomass during this period of rapid decomposition of the simple and readily available plant constituents. Decomposition of maize residues under aerobic conditions. Increased soil C mineralization by 10% (69 pg g-l soils) over 150 days of incubation. The residue addition activated about 9% (66 ug g-l soils) of the soil microbial C. This microbial C component subsequently decayed at a more rapid rate (T1/2 = 26 days) compared to that in the unamended soil (Ti = 177 days) and could explain the enhanced soil C degradation (apparent priming). Field studies examined the effects of management on the rate of mineralization of cereal crop residues in a range of Saskatchewan soils across a climatic gradient. 14c, 15N-labelled barley residues (tops and roots) were incorporated into the plough layer of soil contained in microplots (steel cylinders 30 em dia. x 100 em length) under continuous crop and crop-summerfallow rotations. C and N derived from crop residues mineralized more readily under a crop-summerfallow rotation compared to under continuous crop for the first two growing seasons at all three sites. The difference gradually decreased; after four growing seasons management effects were evident only at the most arid site. Continuous cropping significantly reduced losses of residue N (by 20-60%). This can be attributed to more efficient utilization of mineralized residue N by crop uptake and greater retention in the soil. The 15N losses of 15-50% over a four year period were attributed to leaching as well as denitrification. Continuous cropping increased microbial C and N in the plough layer by up to 20% compared to that in the crop-summerfallow rotation. This would have a direct effect on nutrient cycling. Labelled microbial C accounted for 5-12% of the residue C four years after the addition of residue. However, due to internal cycling of labelled N the portion of residue N present as microbial N was significantly higher. Microplots, containing 14c-, 15N- labelled wheat straw, were set out in semi-arid and sub-humid sites. In the short term, mineralization was more intense at the more moist site. However, the residue C and N at this site was more stable in the long term. The C and N derived from added crop residue decomposition are present as microbial compounds, undecomposed material and humic substances. The C and N derived from from microbial products were more resistant than the C and N derived from crop residues, indicating that some of the undecomposed straw constituents were mineralizing more readily than microbial products. Estimation of soil microbial biomass C and N utilizing the chloroform fumigation-incubation method (CFIM) requires calibration, first for calculation of the flush and, second for determination of the proportions of microbial C (kc) and N (kN) mineralized in fumigated soils during incubation. The' activity associated with C and N mineralization during CFIM can be attributed mainly to proliferating microorganisms (aerobic spore-formers) that develop during incubation, although enzymes associated with non-living cells may make a minor contribution. Alteration of the soil microbial population by CHCl3 treatment from one dominated by fungi to one where bacteria predominate makes it difficult to compare the biochemical activity in fumigated and unfumigated soils. Therefore, the flush was calculated from the C02-c evolved and the net NH4 + -N accumulated in the fumigated soil alone. Calibration of the CFIM utilizing the standard 10 day incubation and a temperature of 280C gave values of kC = 0.40 and =kN 0.33. These values were calculated assuming a fungal:bacterial biomass ratio of 3:1, a bacterial C:N ratio of 4.3, and a fungal C:N ratio of 7.3. The proportion of microbial N mineralized during CFIM was determined using the equation kN = 0.93 - 0.091 (microbial C:N ratio). kN can be estimated directly fran the ratio of CO2-C evolved:net NH4+ -N accumulated (Cf : Nf) during the CFIM incubation utilizing the equation kN = 1.86 (Cf : Nf)-0.879. This equation accurately predicted the proportion of microbial N mineralized in fumigated soil for a number of in vitro produced bacteria and fungi having a wide range in their C:N ratios.