Soil N mineralization and microbial biomass carbon affected by different tillage levels in a hot humid tropic

Tillage is known to reduce soil organic carbon (SOC) and increase soil N mineralization, but information on the level of tillage that increases net soil N mineralization and simultaneously maintains a considerable amount of SOC is poorly known. This study investigated the effect of four levels of tillage (15-cm deep by a local made plough) on net soil N mineralization rate (NMR), net nitrification rate (NNR), pools of NO3−-N and NH4+-N, and microbial biomass carbon (MB-C), water content of soil (WCS) and soil temperature (ST) in a Dystric fluvisols in the hot humid tropical climate of South Andaman Island of India. We hypothesized that: (1) tillage would increase NMR and reduce amount of SOC. But, these changes would depend on frequency of the tillage, i.e. greater would be the tillage frequency; higher, the NMR and decline in the amount of SOC; (2) low tillage would increase NMR, but reduce SOC nearly equal to short term zero tillage. Tillage levels included: (1) long term zero till (not tilled from 1983 to 2002; then from 2003 to 2006 crops (maize–okra rotation) were sown by dibbling, and weeds were cut and mulched), (2) frequent till (tilled three times before each crop sowing in the crop rotation from 1983 to 2002 and 2003 to 2006 as well; weeds were removed), (3) low till (not tilled from 1983 to 1999; then tilled once before each crop sowing in the crop rotation from 2000 to 2002 and weeds were removed; from 2003 to 2006 tilled like 2000–2002, but weeds were uprooted and buried in situ), and (4) short term zero till (from 1983 to 2002 tillage history was the same as in the low till; from 2003 to 2006 the crops were sown by dibbling in the crop rotation and weeds were cut and mulched in situ). Maize (Zea mays L.) was cultivated during wet season (WS, May to October) and okra (Abelmoschus esculentus L.) during post-wet season (PWS, November to January) in all tillage treatments. Soils were sampled in all tillage treatments (levels) across the WS, PWS and dry (DS, February to April) seasons over two annual cycles (2004–2005 and 2005–2006) and analyses were done for the parameters investigated. We found that WCS was the highest (44–48%) during the WS and the lowest (10–16%) during the DS, however, ST was the lowest (25.5–26.5 °C) during the WS and the highest (30.5–33.4 °C) during the DS in all tillage treatments. Across the tillage levels, NMR increased from 1.06 to 1.96 μg g−1 day−1 and NNR from 1.21 to 1.88 μg g−1 day−1, and pools of NO3−-N and NH4+-N from 3.98 to 11.1 μg g−1 and 24.76 to 42.51 μg g−1, respectively. The increase was, however, the highest in the frequent till and the lowest in the long term zero till treatment. The NMR and NNR were the lowest (0.53–0.93 μg g−1 day−1 and 0.49–0.86 μg g−1 day−1, respectively) during the WS and the highest (1.09–1.71 μg g−1 day−1 and 1.06–1.61 μg g−1 day−1) during the PWS in all tillage treatments. The NMR was positively correlated with the MB-C in all tillage treatments. Concurrent with the increase in the NMR, the SOC declined in all tillage treatments, but the decline was the highest in the frequent till and the lowest in the long term zero till treatment. Across the tillage treatments, the MB-C was correlated to the SOC. The SOC in the low till (7.9 mg g−1) treatment was nearly equal to that in the short term zero till treatment (8.8 mg g−1), but NMR was higher (0.86 μg g−1 day−1) particularly during the WS when plant's demand for N is usually high. Our results supported both the hypotheses, and suggested that low tillage might be a good option for soil fertility maintenance and carbon stock build-up in the soils of the hot humid tropics.