Response of nitrogen transformations to moisture changing in red soil

Abstract

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 15N paired labeling technique and a numerical optimization model, the gross conversation rates of the main N transformation processes including organic N mineralization, \begin{document}${\rm{NH}}_4^{+} $\end{document} microbial assimilation, autotrophic nitrification, heterotrophic nitrification, \begin{document}${\rm{NO}}_3^{-} $\end{document} consumption in red soil under different water conditions (20%, 60%, 80%, 100% water holding capacity) were measured and fitted. Result The responses of different types of soil N transformation to moisture change varied substantially. With the increase of soil moisture (from 20% to 100% water holding capacity), the gross mineralization rate of labile organic N ( \begin{document}$M_{{\rm{N}}_{\rm{lab}}} $\end{document} ) increased from 1.757 to 2.598 mg·kg−1·d−1, while the gross mineralization rate of recalcitrant organic N ( \begin{document}$M_{{\rm{N}}_{\rm{rec}}} $\end{document} ) remained stable, and the total gross rate of N mineralization ( \begin{document}$M_{{\rm{N}}_{\rm{lab}}} $\end{document} + \begin{document}$M_{{\rm{N}}_{\rm{rec}}} $\end{document} ) was significantly enhanced by increasing soil moisture from 20% to 100% water holding capacity. The gross rate of autotrophic nitrification ( \begin{document}$O_{{\rm{NH}}_{4}} $\end{document} ) increased significantly with increasing moisture, and reached the maximum value (0.266 mg·kg−1·d−1) at 100% water holding capacity. The gross rate of heterotrophic nitrification ( \begin{document}$O_{{\rm{N}}_{{\rm{rec}}}} $\end{document} ) firstly increased and then decreased with the increase of soil moisture, reaching the maximum value (0.115 mg·kg−1·d−1) at 60% of the maximum water holding capacity. \begin{document}$O_{{\rm{N}}_{{\rm{rec}}}} $\end{document} value (0.115 mg·kg−1·d−1) was lower than that of \begin{document}$O_{{\rm{NH}}_{4}} $\end{document} when soil moisture was 80% and 100% water holding capacity, and the total gross rate of N nitrification ( \begin{document}$O_{{\rm{NH}}_{4}} $\end{document} + \begin{document}$O_{{\rm{N}}_{{\rm{rec}}}} $\end{document} ) increased continuously with the increase of soil moisture. The gross rate of \begin{document}${\rm{NH}}_4^{+} $\end{document} microbial assimilation rate ( \begin{document}$I_{{\rm{NH}}_{4}} $\end{document} ) increased linearly with increasing moisture, and the highest rate (1.941 mg·kg−1·d−1) was observed at 100% water holding capacity. The gross rate of \begin{document}$ {\rm{NO}}_3^{-} $\end{document} consumption ( \begin{document}$C_{{\rm{NO}}_{3}} $\end{document} ) increased significantly at 80% and 100% water holding capacity, and the total inorganic N consumption rate ( \begin{document}$I_{{\rm{NH}}_{4}} $\end{document} + \begin{document}$C_{{\rm{NO}}_{3}} $\end{document} ) 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. [Ch, 5 fig. 1 tab. 48 ref.]