Shrub encroachment increases soil carbon and nitrogen stocks in temperate grasslands in China

Abstract

Shrub encroachment has significant impacts on carbon (C) and nitrogen (N) dynamics of grassland ecosystems, but, how it influences soil organic C (SOC) and soil total N (STN) stocks of grasslands is not well documented, especially in deep soils. Here, we examined these effects by investigating the SOC density (SOCD) and the STN density (STND) at depths of up to 5 m in shrub patches and grassy matrix at 18 grassland sites in Inner Mongolia, China. Our results revealed that 44.0–54.1% of SOC and 65.5–74.8% of STN were stored in soil layers of 100 cm to 500 cm, and that shrub encroachment resulted in a significant accumulation of SOC and STN in shrub patches. The mean SOCD at 0–500 cm soil layers in shrub patches and grassy matrix was 11.34 and 8.39 kg C m−2, respectively, and the mean STND was 2.45 and 2.13 kg N m−2, respectively. These differences between the shrub patches and grassy matrix were equivalent to an increase in the mean SOC and STN stocks of 2.95 kg C m−2 and 0.32 kg N m−2, respectively. Increases in SOCD and STND mainly occurred in 30–300 cm soil layers, and varied between typical grassland and desert grassland. The changes in SOCD and STND exhibited significant negative correlations with temperature and pH and positive correlations with precipitation and plant biomass. Our findings suggest that shrub encroachment can significantly accumulate SOC and STN in deep soils in temperate grasslands, which helps to understand soil C and N dynamics in shrub‐encroached grasslands.

Shrub encroachment decreases soil inorganic carbon stocks in Mongolian grasslands

“…Second, the acidification caused by the accumulation of soil organic matter may have driven the formation of secondary carbonates, that is, causing soil carbonate to dissolve in the surface soil and precipitate in deeper soil layers. Indeed, shrub encroachment in this study area has caused a significant increase in SOC stocks (Li et al, ). Thus, the conversion of primary carbonate to secondary carbonate and the redistribution of SIC in the vertical direction may occur in the study area.…”

“…First, shrub encroachment alters the quantity and quality of plant residues. In alkaline environments, plant residues can reduce soil pH by nitrification (Binkley & Richter, ), and shrub encroachment could significantly increase the amount of biomass and plant residues (Li et al, ). In particular, consistent with most SEGs around the world (Eldridge et al, ), the dominant shrub species in Inner Mongolian SEGs are legumes, and the nitrogen fixation by legumes may lead to a higher rate of nitrogen accumulation in plants than in other herbaceous species (Tang, Unkovich, & Bowden, ) and consequently accelerate the nitrification process (Sanderman, ).…”

Effects of shrub encroachment on soil aggregates and organic carbon vary in different grasslands in Inner Mongolia, China

“…In addition, the higher soil moisture contents in the grass matrix than in the shrub patches were also attributed to high rainfall interception by the shrub canopy, lower water infiltration, and elevated runoff in the shrub patches than in the grass matrix (Ravi et al 2007, Li et al 2013. These factors restrict the growth of shallow herbaceous roots in shrub patches (Zhou et al 2019), which could decrease SOM inputs and reduce the organic carbon content of surface soil aggregates. In the typical grasslands, the soil TN, soil TP, and SMC were greater than those in the desert grasslands (Appendix S1: Fig.…”

“…The TC and TN contents were determined by an elemental analyzer (model PE2400, Perki-nElmer, Waltham, Massachusetts, USA), and the SIC was measured with a carbonate content analyzer (Eijkelkamp 08.53, Giesbeek, The Netherlands). The SOC (% concentration) content of the bulk soil and aggregates were obtained by subtracting the SIC from the TC, and the soil bulk density and SOC density (kg C/m 3 soil) were measured and estimated according to the method described by Li et al (2019).…”

Changes in levels of enzymes and osmotic adjustment compounds in key species and their relevance to vegetation succession in abandoned croplands of a semiarid sandy region

“…The Ordos Plateau is a typical Chinese ecological transition zone that may be classified as an agriculture and animal husbandry ecotone, and because of the influence of overgrazing, land reclamation, and climatic change, the zonal vegetation ( Stipa bungeana ) has been severely degraded, and a semishrub species ( Artemisia ordosica ) has become dominant (Cai et al, ). There are some reports related to grass and shrub interactions, such as grass or shrub dominance are at alternative stable states in the processes of vegetation succession (Peng et al, ), climate and native grassland vegetation significantly influenced the community structures of grasslands with high abundance of shrub (Chen et al, ), cover of shrub patches significantly influenced the community structures, evapotranspiration and soil organic carbon of grassland ecosystems (Li et al, ; Wang et al, ; Zhou et al, ). Grassland degradation on the Ordos Plateau might have originated during the Yuan Dynasty (600–700 years ago), and during this long history, the plant community composition and soil physical and chemical properties have changed to varying degrees; therefore, the Ordos Plateau may be the ideal location for studying the interactions between shrubs and grass (Cai et al, ).…”