Effects Of Different Countermeasures On Soil N Transformations In Typical Subtropical Soils

The characteristics of N transformation in humid subtropical soils vary greatly due to the wide variations of soil properties,particularly soil pH.The properties of the zonal mature soils are adjusted with their natural environment and N can potentially be conserved effectively in thse soils.In contrast to the zonal mature soil occurring in humid subtropical regions,the majority of non-zonal purple soils are neutral or alkaline.High soil pH in purple soils is associated with large nitrification rates and a predominance NO3--N.Nitrate leaching and runoff are key pathways of N loss and an important nitrogen pollution source in the areas where purple soils occur.In this study,a zonal soil(JX soil,pH 5.26)and a non-zonal alkaline soil(SC soil,pH 7.62)collected from the humid subtropics in China were chosen as the objects to investigate the effects of different countermeasures on the soil nitrogen transformation,N2O emissions and nitrogen(N)fertilizer use efficiency(NUE)via incubation experiments and pot experiment.The effects of nitrapyrin,rice straw and its biochar applications on N dynamics at the beginning stage(first 24 hours)of JX soil and SC soil were investigated via a 15N tracing laboratory incubation.Incorporation of rice straw affected almost all processes involved in NH4+ and NO3-production and consumption,and decreased the net mineralization and nitrification rates in both soils significantly.Nitrapyrin decreased the net nitrification rates in both studied soils by affecting different N transformation processes.In the SC soil,nitrapyrin decreased the net nitrification rate by inhibiting the gross autotrophic nitrification rate while in the JX soil the gross autotrophic nitrification was negligible and nitrapyrin decreased its net nitrification via a stimulation of the gross rate of dissimilatory NO3-reduction to NH4+(DNRA).Biochar stimulated the gross rates of mineralization,NH4+ immobilization,autotrophic nitrification and DNRA significantly,but had no significant effect on the net mineralization and nitrification rates in both soils.Countermeasures to regulate N transformations are tightly linked to the prevailing soil N transformation characteristics and should to be taken into account for the recommendation of suitable management options.Incorporation of crop residues is considered as an effective practice to reduce NO3-loss.In the present laboratory study,we compared the effects of alfalfa,rice straw,and sugarcane bagasse on gross N transformation turnover in the SC soil compared with those in the JX soil,at 12 h,3 months,and 6 months after residue incorporation.The gross N transformation rates were determined by 15N tracing.All tested crop residues stimulated the gross N mineralization rates,but reduced the net mineralization rates in both soils at 12 h after residue incorporation;however,the extent of the effect varied with the crop residue qualities,with rice straw having the strongest effects.Crop residues reduced net nitrification rates by depressing gross autotrophic nitrification rates and stimulating NO3-immobilization rates in the SC soil,particularly after rice straw incorporation(net nitrification rate decreased from 16.72 mg N kg-1 d-1 in the control to-29.42 mg N kg-1 d-1 at 12 h of residue incorporation);however,crop residues did not affect the gross autotrophic nitrification rates in the JX soil.Crop residue effects subsided almost completely within 6 months,with sugarcane bagasse showing the longest lasting effects.The results indicated that crop residues affected the N transformation rates in a temporal manner,dependent on soil properties and residue qualities.A laboratory study was conducted to research the effects of different countermeasures on soil N2O emissions.The results showed that nitrapyrin significantly reduced N2O emissions in both soils during the first stage(24 h)after addition,especially in the SC soil where the N2O emission was reduced by 62%.Biochar application significantly enhanced N2O emission in the JX soil,but its effects in the SC soil were not significant.In contrast,the effects of crop residue incorporation on soil N2O emissions depended on the type and characteristics of the crop residues,the physicochemical properties of the soil,and how long the residues had been incorporated into the soil.Over the short term(24 h),alfalfa and rice straw significantly enhanced N2O emissions,whereas bagasse addition did not lead to any significant increases in N2O emissions from the SC soil.The N2O emissions in the JX soil were significantly enhanced by rice straw application,but significantly inhibited by alfalfa and bagasse applications.Increasing the duration of incorporation weakened the effects of alfalfa and rice straw on N2O emissions,whereas the inhibition effect of bagasse on N2O emissions could be maintained for 6 months.Overall,these results highlight the necessity to connect N2O mitigation strategies with soil types and properties when attempting to reduce N2O emissions.The effects of fertilizer,nitrapyrin addition,rice straw and bagasse incorporation on the maize growth and NUE in the SC soil were studied via a pot experiment.Results indicated that crop residues incorporation in the early stages of maize growth could result in the occurrence of N immobilization,causing the maize grew weak.Although crop residues decomposed quickly later,the NUE were still the lowest.The addition of nitrapyrin also had no positive effect on the NUE in the SC soil.In conclusion,when utilize the countermeasures to regulate soil nitrogen transformation,soil properties,the prevailing N transformation characteristics and the regulation targets should be taken into account to select appropriate countermeasures.