The soil environment is modified by fertilizer use, tillage and crop rotations. Three treatments were selected from a long-term experiment established in 1963 at INIA La Estanzuela Experimental Station located in Uruguay (34oS, 57oW), to examine several aspects of soil nitrogen dynamics in different moments. The experiment is located in an Argiudoll soil with moderate slope (3-4 %). Total N (TN), N in particulate organic mater (N-POM), the N-supply capability determined by anaerobic incubation (PMN) and N mineralization rates were evaluated. Selected treatments were: CS-1 (continuous cropping without fertilizer); CS-2 (continuous cropping with N and P fertilizer) and CS-5, a six-year crop – pasture rotation with 50 % of the time under crops and 50 % under grass-legume pastures. Conventional tillage was used throughout the experiment. Crop rotations including pastures (CS-5) showed the highest TN levels due to extra N inputs through biological nitrogen fixation and a reduction in soil erosion. Total nitrogen balances showed approximately an increase of 500 kg ha-1 at the arable layer (20 cm depth) during the pasture phase and a decline of the same magnitude during the cropping phase suggesting low N use efficiency by residual crops and N losses from the system. On average, a production of 25 kg/ha of legume dry matter was required to fix 1 kg N/ha. Inclusion of pastures (CS-5) also resulted in significantly higher values of N-POM compared to continuous cropping (CS-1 and CS-2). At the 0-7.5 cm depth, N-POM was 360 % higher under CS-5 than in CS-1. The treatment that includes grass-legume pastures exhibited also the highest PMN values with a highly significant correlation with N-POM. Overall, both N-POM and PMN were more sensitive indicators to detect the effect of management practices than TN. Similar results were observed for N mineralization rates.
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