Pulse crops grown in prairie crop rotations can provide greater carbon (C) inputs than non-pulse crops in rotation and reduce nitrogen (N) fertilizer requirements. The aim of this research was to estimate the aboveground (ABG) and belowground (BG) partitioning of C and N inputs to soil from continuous (three year) chickpea (CP), lentil (L) and pea (P) systems and from CP, L and P grown in rotation with mustard (M) or wheat (W). Stable isotope techniques were used to label plants grown in a greenhouse and track residue C and N inputs to the bulk soil, heavy fraction organic matter (HF), light fraction organic matter (LF), very light fraction organic matter (VLF), water extractable organic matter (WEOM), the soil microbial biomass (SMB) and the inorganic N pool. Repeat-pulse 13CO2-labeling and shoot 15N-labeling techniques revealed rhizodeposition of C and N was higher in non-continuous pulse crop systems (P-M-CP, P-W-CP, CP-W-CP, L-W-L, P-M-P and P-W-P), than in continuous CP, L and P. Belowground residue (roots and rhizodeposits) C made up 35%, 30% and 33% of total residue C in the continuous CP, L and P, respectively. Belowground residue C made up 50%, 43% and 25% of total residue C in CP, L and P in rotation with M or W, respectively. Belowground-N made up a greater proportion of total residue N than ABG-N in the continuous CP (56%), L (53%) and P (68%) systems, and in the non-continuous CP (76%), L (70%) and P (62%) rotations. Soil pool C and N did not differ between continuous CP, L or P, nor did it differ between the non-continuous CP, L or P rotations. There were no differences between M and W, as the ABG and BG residue C and N in the M pulse crop rotations did not differ from that of the W pulse crop rotations. There was a greater amount of C derived from rhizodeposition (CdfR) and N derived from rhizodeposition (NdfR) in the bulk soil and in the very light fraction organic matter (VLF) of the non-continuous pulse crop rotations, than in the continuous pulse crop systems. This research demonstrates the importance of BG inputs of C and N to soils from CP, L and P grown in rotation with M and W.

Abstract Data collated from around the world indicate that, for every tonne of shoot dry matter produced by crop legumes, the symbiotic relationship with rhizobia is responsible for fixing, on average on a whole plant basis (shoots and nodulated roots), the equivalent of 30-40 kg of nitrogen (N). Consequently, factors that directly influence legume growth (e.g. water and nutrient availability, disease incidence and pests) tend to be the main determinants of the amounts of N2 fixed. However, practices that either limit the presence of effective rhizobia in the soil (no inoculation, poor inoculant quality), increase soil concentrations of nitrate (excessive tillage, extended fallows, fertilizer N), or enhance competition for soil mineralN (intercropping legumes with cereals) can also be critical. Much of the N2 fixed by the legume is usually removed at harvest in high-protein seed so that the net residual contributions of fixed N to agricultural soils after the harvest of legume grain may be relatively small. Nonetheless, the inclusion of legumes in a cropping sequence generally improves the productivity of following crops. Whilesome of these rotational effects may be associated with improvements in availability ofN in soils, factors unrelated to N also play an important role. Recent results suggest that one such non-N benefit may be due to the impact on soil biology of hydrogen emitted from nodules as a by-product of'N, fixation.

Abstract Data collated from around the world indicate that, for every tonne of shoot dry matter produced by crop legumes, the symbiotic relationship with rhizobia is responsible for fixing, on average on a whole plant basis (shoots and nodulated roots), the equivalent of 30-40 kg of nitrogen (N). Consequently, factors that directly influence legume growth (e.g. water and nutrient availability, disease incidence and pests) tend to be the main determinants of the amounts of N2 fixed. However, practices that either limit the presence of effective rhizobia in the soil (no inoculation, poor inoculant quality), increase soil concentrations of nitrate (excessive tillage, extended fallows, fertilizer N), or enhance competition for soil mineralN (intercropping legumes with cereals) can also be critical. Much of the N2 fixed by the legume is usually removed at harvest in high-protein seed so that the net residual contributions of fixed N to agricultural soils after the harvest of legume grain may be relatively small. Nonetheless, the inclusion of legumes in a cropping sequence generally improves the productivity of following crops. Whilesome of these rotational effects may be associated with improvements in availability ofN in soils, factors unrelated to N also play an important role. Recent results suggest that one such non-N benefit may be due to the impact on soil biology of hydrogen emitted from nodules as a by-product of'N, fixation.

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