Effect of conservation tillage and fertilization on photosynthetic physiology of soybean at full-bloom stage

Based on field plot experiments in the Ansai region, Shaanxi Province, the paper analyzed net photosynthetic rate, stomatal conductance and water-use efficiency in soybean leaves at full-bloom stage under different tillage and fertilization. The LI-6400 portable photosynthesis system was used in the analysis. The treatments included plow tillage with chemical fertilizer (CF), plow tillage with organic fertilizer (CM), non-fertilized plow tillage (CN), fertilized no-tillage (NF), no-tillage with organic manure (NM) and non-fertilized no-tillage (NN). The results show that diurnal variation of net photosynthetic rate of soybean follows the typical bimodal curve in the different treatments; peaking at 11:30 and 16:00. NM and NF treatments, especially NM, significantly enhance net photosynthetic rate of soybean. A significant correlation exists between stomatal conductance and net photosynthetic rate (P<0.01), with a correlation coefficient of 0.999 4. Diurnal variation of stomatal conductance is also a bimodal; peaking at 11:30 and 16:00. In different treatments, water-use efficiency of soybean follows a uni-peak curve; highest at 10:00 and lowest after 13:00. NM and NF treatments have a significantly enhancing effect on soybean water-use efficiency. Correlation analysis shows that stomatal conductance (Cond>), evaporation ratio (Tr), leaf temperature (Tl), photosynthetically active radiation (PAR) and supplemental vapor pressure deficit (Vpdl, based leave temperature) are positive factors of photosynthesis in soybean. On the other hand, inter-cell CO₂ concentration (Ci), air CO₂ concentration (CO₂R ) and air relative humidity (RH-R) mainly limit photosynthesis in soybean. CF & NF at 8:30~10:20 and 13:00; CM & NN at 8:30, 11:30~13:00 & 17:30; and CN & NM at 8:30 & 13:30 have significant correlation between impacting factors and net photosynthetic rate, with a correlation coefficient of ±0.9. The optimum management modes for growing soybean in dry farmlands on the Hilly Loess region are NM and NF.