Performance of key legumes under deteriorating soil water conditions: Effect of drought on the contribution by legumes to soil N fertility in Central Queensland

The incorporation of legumes into crop rotations has been suggested as a strategy to address declining soil fertility, especially of nitrogen (N), in Central Queensland cropping systems. However, traditional methods to determine the contribution made by legumes to soil N are flawed in that usually only the above ground biomass (AGB) or at best macro root tissues are assessed. Furthermore crops regularly experience soil water deficits in this environment, but the impact this has on the contribution made to soil fertility by legumes is poorly documented.

A published ¹⁵N foliar labelling methodology that accurately estimates legume below ground biomass N (BGB-N), was adapted under polyhouse conditions and verified in the field for lab lab, lucerne and siratro. For the legumes investigated, labelled macro root tissue at a depth of 10-20 cm was representative of recoverable root tissue (all > 500 um) throughout a 60 cm soil profile. A protocol was established that reliably estimated the ¹⁵N content of soil enriched by the labelled root system to within a 95% confidence interval.

N₂ fixation in symbioses which translocate fixed N as amides from their nodules are reported to have a greater tolerance to water stress than that in symbioses which export ureides. The impact of water stress on N₂ fixation and the distribution of biomass and N between above and below ground compartments was therefore studied for two commercially important legumes: mungbean (a ureide exporter) and peanut (an amide exporter). Under glasshouse conditions, drought had no effect on mungbean BGB-N as a proportion of legume N (AGB-N + BGB-N) (35%) whereas the proportion of peanut N as BGB-N increased from 33 % under optimal conditions to 44% (P<0.05) under terminal drought. Legume N and N derived from atmospheric N2 (Ndfa), including BGB-N and BGB Ndfa was significantly greater than estimates based on AGB-N alone. Under drought, whole plant Ndfa was twice that of values based on AGB-N alone. The proportion of cereal crop N derived from legume BGB-N (27%) was significantly greater than that derived from AGB-N (20%). The proportion of mungbean N derived from atmospheric N₂ (%Ndfa) was 51% under optimal soil water conditions but declined to 20 and 12% under the moderate and terminal droughts, respectively. Severe drought produced a small but significant reduction in N₂ fixation activity compared to optimally watered plants (%Ndfa decreased to 67 from 78%; P<0.05). Moderate drought had not significant effect on %Ndfa compared to optimally watered plants (78 and 75%Ndfa, respectively).

Similar trends to those reported in the glasshouse trial were observed under field conditions, with significantly greater contributions by mungbean and peanut to soil N fertility and cereal crops documented than reported previously for a range of grain and pasture legumes used in Central Queensland cropping systems. Droughted legumes increased the proportion of BGB-N at depth (20-60 cm), with greater increases by peanut. Despite drought induced decreases in mungbean and peanut %Ndfa (at 59 and 32%, respectively, of irrigated plants), droughted mungbean and peanut %Ndfa was still substantial (30 and 53%, respectively). Droughted whole mungbean and peanut Ndfa was estimated at 10 and 21 kg N/ha, respectively. However, net N balance was nevertheless neutral to negative for mungbean when N exported in harvested grain was considered. The estimates of Ndfa were three fold higher than that based on AGB-N only, and were consistent with the thesis that legumes possessing an amide exporting symbiosis offer significantly greater potential to improve soil N fertility in the region. The drought survival mechanisms exhibited by legumes is also a factor determining whether or not accumulated N and N₂ fixation were maintained under increased soil water deficit and when a net contribution to soil N stores was likely.