Microbial Biomass Is More Important than Runoff Export in Predicting Soil Inorganic Nitrogen Concentrations Following Forest Conversion in Subtropical China

Chao Xu,Zhijie Yang,Liuming Yang,Minhuang Wang,Shidong Chen,Xiaofei Liu,Teng-Chiu Lin,Yusheng Yang,Jr-Chuan Huang,Decheng Xiong

doi:10.3390/land11020295

Abstract

Elevated runoff export and declines in soil microbial biomass and enzyme activity following forest conversion are known to reduce soil inorganic nitrogen (N) but their relative importance remains poorly understood. To explore their relative importance, we examined soil inorganic N (NH4+ and NO3−) concentrations in relation to microbial biomass, enzyme activity, and runoff export of inorganic N in a mature secondary forest, young (five years old) Castanopsis carlessi and Cunninghamia lanceolate (Chinese fir) plantations, and forests developing through assisted natural regeneration (ANR). The surface runoff export of inorganic N was greater, but fine root biomass, soil microbial biomass, enzyme activity, and inorganic N concentrations were smaller in the young plantations than the secondary forest and the young ANR forests. Microbial biomass, enzyme activity, and runoff inorganic N export explained 84% and 82% of the variation of soil NH4+ and NO3− concentrations, respectively. Soil microbial biomass contributed 61% and 94% of the explaining power for the variation of soil NH4+ and NO3− concentrations, respectively, among the forests. Positive relationships between microbial enzyme activity and soil inorganic N concentrations were likely mediated via microbial biomass as it was highly correlated with microbial enzyme activity. Although surface runoff export can reduce soil inorganic N, the effect attenuated a few years after forest conversion. By contrast, the differences in microbial biomass persisted for a long time, leading to its dominance in regulating soil inorganic N concentrations. Our results highlight that most of the variation in soil inorganic N concentration following forest conversion was related to soil microbial biomass and that assisted natural regeneration can effectively conserve soil N.

Highlights

Large areas of natural forests have been replaced by forest plantations around the world
The activity of the enzymes was lower in the young plantations than in the mature secondary forest and young assisted natural regeneration (ANR) forests except that there was no significant difference in BG activity among different forest types and no significant differences in acid phosphatase (AP) activity between the Chinese-fir plantations, the mature secondary forest and the young ANR forests (Figure 3)
No significant differences were found between the mature secondary forest and the young ANR forests and no significant differences were found between the two types of young forest plantation in the activity of any of the five enzymes (Figure 3)

Summary

Introduction

Large areas of natural forests have been replaced by forest plantations around the world. A large number of studies have examined the effects of forest conversion on important ecosystem services and processes [5,6,7]. The conversion of natural forests to forest plantations often causes soil degradation [8,9,10], especially the decline of nitrogen (N) availability [11,12,13]. Revealing key processes leading to the decline of soil inorganic N availability is critical to probe into the decline of net primary productivity and soil fertility following forest conversion [18,19,20]

Methods

Results

Discussion

Conclusion