Shrub Encroachment Shapes Soil Nutrient Concentration, Stoichiometry and Carbon Storage in an Abandoned Subalpine Grassland

Leilei Ding,Wen Zhang,Xin Wei,Yu Zhang,Fuli Yang,Xi Chen,Shige Li,Puchang Wang,Yujun Zhang

doi:10.3390/su11061732

Leilei Ding, Wen Zhang + Show 7 more

Open Access

https://doi.org/10.3390/su11061732

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Abstract

Soil nutrient stoichiometry plays a substantial role in terrestrial carbon and nutrient cycling. However, the changes in soil nutrient stoichiometry with shrub encroachment (SE) remain poorly understood, especially in subalpine areas. We examined the changes in soil nutrient concentration, nutrient stoichiometry, and organic carbon (OC) storage (at a depth of 0–5, 5–10 and 10–20 cm) in three successional shrub encroachment stages (early, mid and late) in an abandoned subalpine Eulalia pallens (Hackel) Kuntze grassland. An ANOVA showed that SE did not produce serious soil acidification, but significantly increased the soil OC and total phosphorous (TP) concentration, and improved the stoichiometry ratio of soil OC to total nitrogen (OC:TN) in all layers. OC storage tended to increase with SE. SE thus did not indicate degradation of the grassland. A redundancy analysis (RDA) and partial RDA revealed that the shrub relative cover and soil water content were the most important factors affecting the soil nutrient concentration, that the soil available phosphorous (AP), nitrogen, potassium, calcium (ACa), and magnesium concentration and shrub relative cover were the most important factors influencing soil nutrient stoichiometry ratios, and that soil OC:TN, TN:TP, OC:TN:TP, and AP:ACa ratios, bulk density, and pH were the most important factors influencing soil OC storage over SE. Our study provides insights into SE in grassland areas, and potentially provides a useful reference for ongoing grassland conservation and restoration in subalpine regions.

Highlights

Shifts from grassland to shrubland, the thicketization of grassland, or the encroachment of woody plants into grassland—a phenomenon known as shrub encroachment (SE)—occur all over the world [1,2,3,4,5,6] and have taken place during the last century [3,4,5,7,8], and even in the past two centuries [9,10]
OC was determined using an elemental analyzer (Elementar total organic carbon/total nitrogen analyzer, Germany); TN was determined through sulfuric acid digestion and using an FOSS automatic nitrogen determination apparatus; available nitrogen (AN) was analyzed by the alkali diffusion method; total phosphorus (TP) was determined by NaOH digestion followed by ammonium-molybdate colorimetry (T181 XinShiJi ultraviolet spectrophotometer, China); available phosphorous (AP) was extracted using the NaHCO3-ultraviolet spectrometer subsystem (T181 XinShiJi ultraviolet spectrophotometer, China); AK was extracted with ammonium acetate; and available calcium (ACa) and available magnesium (AMg) were determined using a soil:water (1:5) mixture by an Atomic Absorption Spectrophotometer
OC and TN were expressed as g·kg−1; TP, AN, AP, AK, ACa and AMg as mg·kg−1; water content (WC) was expressed as a decimal; bulk density (BD) as g·cm−3; the soil stoichiometry as molar ratios; and the C storage (CS) was calculated for the 0–5 cm, 5–10 cm, 10–20 cm, and 0–20 cm layers according to the following formula [61], and expressed as kg·m−2: n

Summary

Introduction

Shifts from grassland to shrubland, the thicketization of grassland, or the encroachment of woody plants into grassland—a phenomenon known as shrub encroachment (SE)—occur all over the world [1,2,3,4,5,6] and have taken place during the last century [3,4,5,7,8], and even in the past two centuries [9,10]. The effects of SE on soil nutrient concentration and stoichiometry in natural, abandoned subalpine Eulalia pallens (Hackel) Kuntze (E. pallens or Baijiangan in Chinese) grassland have not yet been explored. Understanding and evaluating the consequences of SE on soil nutrient concentration and stoichiometry in this subalpine E. pallens grassland will be key to improving our knowledge, protecting species diversity, and developing good land management and policies for improving soil fertility and C sequestration. (3) What are the implications of changed soil nutrient concentration, stoichiometry, and CS caused by SE in this special area?

Site Description

Data Analyses

Results

SE in the Study Area May Not Indicate Land Degradation

The Main Factors Controlling Soil Nutrients Over Shrub Encroachment

The Main Factors Changing Soil Nutrient Stoichiometry Over Shrub Encroachment

The Main Factors Controlling Soil Carbon Storage over Shrub Encroachment