Session 10: Conservation Agriculture: Key to Intensifying Crop Production

Background Nitrogen (N) is the most limiting nutrient in crop production worldwide. Due to this limiting nature of N, its management poses certain challenges in crop production. In high input commercial systems, N fertilizer is often over applied therefore making N a potential pollutant in ground water through nitrate leaching while in low input subsistence systems N is underutilized, resulting in lower yields. These N fertilization management challenges mandate that alternative sources of N in crop production be sought. Biologically fixed N either through free living or symbiotic N-fixers may help to reduce the need for N fertilization in high input systems whereas in the low input systems it may help to augment the low yields without added costs. To this end, some agronomic practices have been developed including the no-till, cover crops and crop rotations. As these practices are being adopted by farmers worldwide, there has to be an understanding of how they interact together to effect the N transformation processes that will determine its use efficiency. Experimental Approach The current study was aimed at determining the effect of different agronomic practices including tillage (no-till and till), cover crops (no cover, hairy vetch and winter wheat) and different rates N fertilizer (0, 34 and 101 kg N/ha) on N transformation processes as measured by net N mineralization and nitrification rates and on the microbial biomass carbon (C) dynamics during cotton growing season. The experiment was set up as randomized complete block design with split-split plot. N fertilizer rate was set up as the whole plot, cover crops as split plot while tillage was the split-split plot. Net N mineralization and nitrification rates were assessed through the use of intact in situ core incubation method (DiStefano and Gholz 1986). A 10-cm long polyvinylchloride (PVC) pipe containing a soil core of 7.5-cm fitted underneath with mixed ion (cation and anion) exchange resins in nylon mesh bag was used. Mixed ion resins were used to adsorb the inorganic N (NH4-N and NO3-N) that was released from the mineralization of the soil organic N or soil organic matter within the PVC pipe. Incubation was carried out twice during the cotton growing season. The first incubation lasted for 82 days (from crop emergence to cotton boll development) while the second incubation lasted for 46 days (from boll development to prior harvest). The net N mineralization rate was calculated as the total sum of NH4-N and NO3-N produced over the incubation period divided by incubation time in days. This sum total included both N from the soil within the PVC pipe and N that was adsorbed on mixed ion resins. Net nitrification rate was estimated as the sum of only nitrate from within the PVC pipe and from the mixed ion resins over the incubation time. For microbial biomass C, soil was sampled four times the during cotton growing season, at crop emergence, flowering, boll development and prior to harvest. Microbial biomass C was determined through chloroform slurry extraction (Ferier and Schimel 2002). Two replicates of the same sample are obtained, together with the extraction solution (KCl); one replicate is