| The importance of soil organic carbon (SOC) and nitrogen (N) sequestration in agricultural soils as climate-change mitigating strategy has received robust attention worldwide in relation to soil management and its effects on soil nutrient balances and flows. Crop rotation in combination with varying tillage intensity can change the quantity and quality of residues which affect nutrient dynamics and ultimately soil fertility and quality. This study was conducted to determine the effects of different tillage systems and residue management on distribution of nutrients (P, K, Mg, Ca, Mg, Na, Fe, Mn, Cu, Zn), SOC fractions, soil pH, soil bulk density, soil porosity, concentration and storage and stratification of SOC and N under wheat (Triticum aestivum L.)-maize (Zea mays L.) double cropping system in the North China Plain (NCP). Four tillage systems for winter wheat established in2001were:moldboard plow tillage with maize residues removed (PTO), moldboard plow tillage with maize residues incorporated (PT), rotary tillage with maize residues incorporated (RT), and no-till with maize residues retained on the soil surface (NT).Compared with PTO and PT, higher SOC and N concentration were observed in the surface layer (0-10cm depth) under NT and RT. The SOC concentration was drastically reduced with increases in soil depth regardless of tillage systems and residue management. Regressive significant (P<0.05) declines were recorded under NT and RT than under PT systems which led to stratification with depth. The distribution of SOC and N was somewhat uniformly distributed in the plough layer due to mechanical tillage operations. The soil bulk density was significantly higher (P<0.05) under NT than under RT and PT systems due to less mechanical operations in the soil compared to complete soil inversion under PT. On the other hand, the soil porosity was significantly higher under PT and PTO compared to NT which had the lowest porosity due to higher bulk density. In general soil bulk density and soil porosity are usually inversely related.The concentrations of SOC fractions followed a decreasing trend with increase in soil depth in76%of the observations and were significantly affected by tillage with time. The results showed that SOC fractions were enhanced under NT and RT at0-10cm depth compared to PT and PTO. However, significant decreases in SOC fractions were observed below10cm depths (P<0.05) regardless of the tillage system imposed. An apparent stratification of SOC fractions (except LFC and NPOC) occurred under NT than under PT and PTO for>10cm depths. The LFC presented an inconsistent trend while both POC and LOC consistently responded to different soil tillage management changes. All parameters were positively correlated (P<0.01) and linear regressions exhibited similar patterns (P<0.01).Higher nutrient concentrations under NT at0-5cm depth were observed for P in all the years, and those of Mn (2009) and N only in2007/2009. Concentration of Zn was significantly higher (P<0.05) under RT at0-10cm and likewise for N at5-10cm depth in all the years. Concentrations of Ca and Na increased with increase in soil depth while those of N, P, K, Mn, Zn and Cu decreased with increase in depth but those of Mg and Fe exhibited a variable pattern in all the years irrespective of tillage systems. Correlation analysis also showed highly variable relations (55pairs) among nutrients and soil pH. Soil pH was not significantly affected by tillage and residue management systems.The initial SOC stocks were lower (P<0.05) under NT and RT than PT systems, however, the opposite trend was observed in2012. Compared with2001, the net profile (0-30cm) SOC sequestration rate was10.60,13.95,13.65, and14.92Mg ha-1in2012under PT0, PT, RT, and NT, respectively. Meanwhile, the trend in profile (0-30,0-50cm depth) N stock was NT>RT>PT>PT0in both years. Similar net profile SOC sequestration (0-30cm) was observed under NT and PT, and likewise under NT and RT treatments for N sequestration.The stratification ratio (SR) of SOC was significantly higher under NT compared to other tillage systems and those of N somewhat followed the similar pattern in most of the years. Significantly higher (P<0.05) SOC:N ratio was observed under PT systems than under RT and NT. In addition the relationship among measured parameters presented an interesting dynamics which validated some of the mechanisms responsible for differences in tillage. For example, the concentration of major plant nutrients (N, P, and K) and Zn were significantly correlated with SOC and its fractions while other nutrients presented a varied trend. The concentration of SOC, N and SOC fractions significantly (P<0.01) increased simultaneously to varying magnitude because of different tillage systems. Soil bulk density was negatively correlated to varying margins with all the measured parameters (SOC, SOC fractions and nutrients).Therefore, in order to enhance soil quality improvements in NCP under different cropping systems, crop residue inputs should be complemented by a proper sustainable tillage and residue management system. Moreover, in the context of yield, nutrient use, integrated fertilizer management under conservation tillage system in NCP region, higher soil fertility and crop yield needs to be attained. |
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