Influence of tillage treatments on N2 fixation of soybean

With the potential of conservation tillage practices to increase soybean grain yields and soil erosion protection, 2 field experiments were established to determine its effect on N2 fixation by soybean. The major tillage practices evaluated were the conservation practices of no-tillage (standing stubble), mulch (stubble slashed) and incorporated late (stubble ploughed once immediately before sowing). These were compared to the conventional tillage practices of repeated stubble incorporation (incorporated early) over a fallow and stubble removal by burning and incorporated (stubble burnt then multiple ploughing over the fallow) and no-tillage burn (stubble burnt). Two types of cropping system were studied: (1) double cropping (opportunity cropping) and (2) cropping after a fallow. N2 fixation was measured by the 15N dilution technique and the ureide method. Conservation tillage (no-tillage, mulch and incorporated late) increased N2 fixation to over 85% of grain N compared to the conventional tillage practices (early incorporation and burn followed by incorporation), where N2 fixation was <75%. N2 fixation was increased in tillage treatments that maintained high amounts of previous crop residues on the soil surface and not by pre-sowing tillage. Conservation tillage increased soybean N2 fixation when sown as a double crop or after a fallow.