Short term changes in dynamics of C and N in soil when crops are cultivated on permanent raised beds

Densely populated, intensively cropped highland areas in the subtropics are prone to erosion and declining soil fertility, making agriculture unsustainable. Permanent raised bed planting systems, as a form of conservation agriculture, have been developed to reduce production costs while conserving resources and sustaining the environment. In 2004, a new experiment with long term focus was started under rain fed conditions at El Batan (Mexico; 2,240 m a.s.l.; 19.31N, 98.50W; Cumulic Phaeozem), which aims at understanding the effects of (1) tillage (conventionally tilled or permanent raised beds), (2) residue management (retention or removal) and (3) N fertilizer application (0 or 120 kg N/ha) on N availability in a yearly maize/wheat rotation system. Incubation experiments were conducted to establish how the different treatments affect C and N dynamics in the soil. Tillage increases the availability of soil organic matter by soil aggregate disruption, enhancing C and N mineralization. Conventionally tilled raised beds with incorporation of crop residues increased the CO2 production rate. In both tillage systems, retention of maize or wheat residue without N fertilizer application led to N immobilization. In permanent raised beds, however, the immobilization due to residue retention could be compensated by application of N fertilizer, while conventionally tilled raised beds appeared to use the applied N fertilizer less efficiently.