红壤氮转化对土壤水分变化的响应

Response of nitrogen transformations to moisture changing in red soil

[目的]土壤水分变化会影响微生物介导的氮转化。探明土壤氮初级转化速率,反映土壤内部氮素动态变化,探索氮转化对土壤水分变化的响应机制。[方法]采用^(15)N成对标记技术,利用数值优化模型,量化不同水分条件(最大持水量的20%、60%、80%、100%)下,有机氮矿化、铵态氮(NH_(4)^(+))微生物同化、自养硝化、异养硝化和硝态氮(NO_(3)^(-))消耗等主要氮转化过程的初级转化速率。[结果]土壤不同氮转化过程对水分变化的响应不同。随土壤含水量上升(从最大持水量的20%升至100%),土壤中易分解有机氮库初级矿化速率(M_(Nlab))从1.757 mg·kg^(-1)·d^(-1)增加到2.598 mg·kg^(-1)·d^(-1),难分解有机氮库初级矿化速率(M_(Nrec))变化不显著,总初级矿化速率(M,即M_(Nlab)和M_(Nrec))显著上升。初级自养硝化速率(ONH_(4))随土壤含水量增加而增加,在最大持水量为100%时达到最大值(0.266 mg·kg^(-1)·d^(-1));初级异养硝化速率(O_(Nrec))随土壤含水量增加先上升后下降,在最大持水量为60%时达到最大值(0.115 mg·kg^(-1)·d^(-1));土壤在最大持水量为80%和100%时ONH_(4)显著大于O_(Nrec),总初级硝化速率(N,即ONH_(4)和O_(Nrec))随土壤含水量增加而增大。总初级NH_(4)^(+)微生物同化速率(INH_(4))随土壤含水量增加线性上升,土壤在最大持水量的100%时达到最大值(1.941 mg·kg^(-1)·d^(-1));初级NO_(3)^(-)消耗速率(C_(NO_(3)))在最大持水量的80%和100%时明显增加,总无机氮消耗速率(I_(NH_(4))和C_(NO_(3)))随土壤含水量增加显著增大,并在最大持水量的80%时超过总氮初级矿化速率。因此,随含水量增加土壤氮净矿化速率先上升到最大值,然后迅速下降为负值。[结论]红壤不同无机氮产生和消耗过程对水分变化的响应不同;适当增加土壤含水量可提高红壤氮素的可利用性。

[Objective]Soil moisture variations can affect microbial-mediated N transformation.The purpose of this study is to determine the gross transformation rate of soil N,and explore the dynamic change of N in soil and the response mechanism of N transformation to soil water change.[Method]By using^(15)N paired labeling technique and a numerical optimization model,the gross conversation rates of the main N transformation processes including organic N mineralization,NH_(4)^(+)microbial assimilation,autotrophic nitrification,heterotrophic nitrification,NO_(3)^(-)consumption in red soil under different water conditions(20%,60%,80%,100%water holding capacity)were measured and fitted.[Result]The responses of different types of soil N transformation to moisture change varied substantially.With the increase of soil moisture(from 20%to 100%water holding capacity),the gross mineralization rate of labile organic N(M_(Nlab))increased from 1.757 to 2.598 mg·kg^(-1)·d^(-1),while the gross mineralization rate of recalcitrant organic N(M_(Nrec)remained stable,and the total gross rate of N mineralization(M_(Nlab)+M_(Nrec)was significantly enhanced by increasing soil moisture from 20%to100%water holding capacity.The gross rate of autotrophic nitrification(ONH_(4))increased significantly with increasing moisture,and reached the maximum value(0.266 mg·kg^(-1)·d^(-1))at 100%water holding capacity.The gross rate of heterotrophic nitrification(O_(Nrec))firstly increased and then decreased with the increase of soil moisture,reaching the maximum value(0.115 mg·kg^(-1)·d^(-1))at 60%of the maximum water holding capacity.O_(Nrec)value(0.115 mg·kg^(-1)·d^(-1))was lower than that of ONH_(4)when soil moisture was 80%and 100%water holding capacity,and the total gross rate of N nitrification(O(NH_(4)^(+)O_(Nrec)increased continuously with the increase of soil moisture.The gross rate of NH_(4)^(+)microbial assimilation rate(INH_(4))increased linearly with increasing moisture,and the highest rate(1.941 mg·kg^(-1)·d^(-1))was observed at 100%water holding capacity.The gross rate of NO_(3)^(-)consumption(C_(NO_(3)))increased significantly at 80%and 100%water holding capacity,and the total inorganic N consumption rate(I_(NH_(4)^(+))C_(NO_(3)))increased obviously,and exceeded total gross rate of N mineralization at 80%water holding capacity.In general,the net rate of N mineralization firstly increased to the maximum value at 60%water holding capacity,and then decreased rapidly to negative value(i.e.net consumption)at 80%and 100%water holding capacity.[Conclusion]Different types of inorganic nitrogen production and consumption have different responses to water change in red soil.The availability of nitrogen in red soil could be improved by increasing soil water content appropriately.