Soil nitrogen (N) availability is a major constraint for plant growth and consequently impacts soil carbon (C) sequestration following afforestation. Soil N constraint for plant growth would become even more aggravated in afforested soils. As over 90% of N is incorporated into soil as organic forms, available N released from soil organic N predominantly determines soil N availability and ecosystem N cycling. However, how afforestation affect soil organic N and whether progressive N limitation will occur as a consequence of additional C sequestration following afforestation remain largely unknown. 

Supervised by Prof. CHENG Xiaoli, FENG Jiao, a postdoctor of Wuhan Botanical Garden, determined N-hydrolyzing enzyme activities, N contents, and the δ¹⁵N values in soil aggregates following 30 years of afforestation in subtropical China.  

Afforestation increased total N (TN) contents and N-hydrolyzing enzyme activities, but reduced the percentages of recalcitrant N in TN and the δ¹⁵N values in soil aggregates. Soil enzymes for N acquisition scaled isometrically with C acquisition with a slope of ~1.0.  

Results demonstrated that N constraint for soil C sequestration could be alleviated by increasing soil N-hydrolyzing enzyme activities, reducing recalcitrant N:TN ratios and sustaining homeostatic ecoenzymatic C:N ratios following afforestation, which led to tight coupling of soil N and C cycling. This accelerated N cycling and tightly coupled soil N and C cycling would be favorable for the maintenance of C sequestration following afforestation. 

This research was financially supported by the National Natural Science Foundation of China and the "Strategic Priority Research Program B of the Chinese Academy of Sciences”. Results have been published in Soil Biology and Biochemistry entitled “Stimulation of nitrogen-hydrolyzing enzymes in soil aggregates mitigates nitrogen constraint for carbon sequestration following afforestation in subtropical China”. 

Diagram of proposed impacts of afforestation on soil N cycling around Danjiangkou Reservoir area in China (Image by CHENG’s group)