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 from 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. 

Results have been published in a paper 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". 

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”.

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