Effects of nitrogen addition on soil active organic carbon in a temperate grassland of Nei Mongol, China

doi:10.17521/cjpe.2023.0148

Abstract

Abstract:

Aims Both the carbon cycle and the function of grassland ecosystem as a carbon sink are impacted by the rising nitrogen deposition. Active organic carbon content is an important measure that can reveal changes in soil carbon pool. For a thorough understanding of carbon cycling and the creation of sensible ecosystem management strategies, it is essential to investigate the impacts of nitrogen addition on the active organic carbon fractions of grassland soils.

Methods Five different nitrogen addition treatments were set up in a temperate typical steppe of Nei Mongol. Soil organic carbon fractions content, soil physical and chemical properties, aggregate stability, microbial activities and extracellular enzyme activities were measured. Pearson correlation and structural equation model (SEM) were used to examine the relationships.

Important findings Nitrogen addition reduced the contents of dissolved organic carbon (DOC), microbial biomass carbon (MBC), and easily oxidizable organic carbon (EOC). The contents of DOC, MBC, and EOC all decreased with the increases of soil depth. The treatment of 5 g·m^-2·a^-1 nitrogen addition significantly promoted the decomposition of active organic carbon fractions. The effect of nitrogen addition on soil active organic carbon fractions content was regulated by biotic (microbial biomass, extracellular enzyme activity, etc.) and abiotic (soil physical and chemical properties, aggregate stability, etc.) factors. Nitrogen addition reduced soil density, increased mean mass diameter and the proportion of large aggregates, increased the contact between organic matter and substrate, promoted the decomposition of active organic carbon, and reduced the contents of DOC and EOC. Nitrogen addition inhibited the activities of polyphenol oxidase and peroxidase, reduced the decomposition of difficult-to-decompose organic matter and the contents of EOC and MBC. Nitrogen addition increased the activities of β-glucosidase and cellulose hydrolase, promoted the utilization of DOC by microorganisms, and reduced the content of DOC. Our results indicated that nitrogen addition treatment can affect the decomposition process of active organic carbon by changing soil physicochemical properties and the secretion of extracellular enzymes from microorganisms, promoting the release of carbon in grassland soils. This provided a theoretical basis for further exploration of grassland soil carbon dynamics under nutrient addition in the future.

Key words: nitrogen addition, active organic carbon, soil aggregates, biotic factor, physicochemical properties

YAN Chen-Yi, GONG Ji-Rui, ZHANG Si-Qi, ZHANG Wei-Yuan, DONG Xue-De, HU Yu-Xia, YANG Gui-Sen. Effects of nitrogen addition on soil active organic carbon in a temperate grassland of Nei Mongol, China[J]. Chin J Plant Ecol, 2024, 48(2): 229-241.

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https://www.plant-ecology.com/EN/Y2024/V48/I2/229

Figures/Tables 8

Fig. 1 Effects of different nitrogen addition treatments on aboveground and belowground biomass of vegetation in a temperate grassland of Nei Mongol (mean ± SE). Different lowercase letters indicate significant differences in belowground biomass between different treatments (p < 0.05), different uppercase letters indicate significant differences in aboveground biomass between different treatments (p < 0.05). N0, N2, N5, N10 and N25 indicate nitrogen addition amounts of 0, 2, 5, 10, 25 g·m-2·a-1, respectively.

Table 1 Effect of different nitrogen additions on soil physical and chemical properties in temperate grassland of Nei Mongol (mean ± SE)

处理 Treatment	土层深度 SLD (cm)	土壤pH Soil pH	土壤密度 SD (g·cm^-3)	土壤含水量 SWC (%)	土壤全碳含量 TC content (g·kg^-1)	全氮含量 TN content (g·kg^-1)	微生物生物量氮含量 MBN content (mg·kg^-1)
N0	0-10	8.22 ± 0.01^Ca	1.26 ± 0.02^Aa	18.27 ± 0.10^Ab	18.40 ± 0.17^Cd	1.39 ± 0.09^Ab	45.03 ± 3.55^Aac
	10-20	8.29 ± 0.00^Ba	1.24 ± 0.02^Aa	17.62 ± 0.27^Bc	22.13 ± 0.29^Ab	1.37 ± 0.09^ABa	34.93 ± 0.35^Bab
	20-30	8.47 ± 0.00^Aa	1.13 ± 0.02^Bb	17.06 ± 0.10^Bb	19.63 ± 0.09^Bd	0.92 ± 0.11^Ba	14.36 ± 0.28^Ca
N2	0-10	8.11 ± 0.00^Cb	1.31 ± 0.01^Aa	19.39 ± 0.19^Ba	21.26 ± 0.10^Bb	1.84 ± 0.22^Aa	53.74 ± 3.20^Aa
	10-20	8.16 ± 0.01^Bb	1.09 ± 0.04^Bb	20.45 ± 0.31^Ab	24.77 ± 0.31^Aa	1.44 ± 0.07^Aa	19.13 ± 3.70^Bc
	20-30	8.29 ± 0.01^Ac	1.09 ± 0.01^Bbc	20.36 ± 0.31^Aa	23.60 ± 0.58^Ab	0.94 ± 0.15^Ba	15.00 ± 0.94^Ba
N5	0-10	8.13 ± 0.01^Bb	1.18 ± 0.01^Ab	19.61 ± 0.30^Ba	21.98 ± 0.05^Ba	1.44 ± 0.03^Aab	31.63 ± 2.56^Ab
	10-20	8.15 ± 0.01^Bb	1.19 ± 0.03^Aab	21.42 ± 0.21^Aa	25.25 ± 0.32^Aa	1.43 ± 0.04^Aa	26.17 ± 1.58^Abc
	20-30	8.37 ± 0.01^Ab	1.24 ± 0.04^Aa	19.53 ± 0.52^Ba	25.10 ± 0.16^Aa	1.16 ± 0.10^Ba	14.14 ± 0.24^Ba
N10	0-10	7.97 ± 0.01^Cc	1.15 ± 0.03^Ab	19.26 ± 0.40^Aa	20.28 ± 0.08^Bc	1.65 ± 0.11^Aab	51.99 ± 2.01^Aa
	10-20	8.01 ± 0.01^Bc	1.03 ± 0.04^Bb	18.25 ± 0.38^Ac	22.65 ± 0.23^Ab	1.47 ± 0.11^Aa	41.81 ± 8.46^Aa
	20-30	8.17 ± 0.01^Ad	1.16 ± 0.02^Ab	19.57 ± 0.41^Aa	22.18 ± 0.04^Ac	0.86 ± 0.08^Ba	14.93 ± 1.72^Ba
N25	0-10	7.89 ± 0.01^Cd	1.17 ± 0.01^Ab	19.99 ± 0.12^Aa	21.36 ± 0.13^Bb	1.60 ± 0.16^Aab	38.19 ± 2.72^Abc
	10-20	8.02 ± 0.00^Bc	1.10 ± 0.01^Bb	19.74 ± 0.12^ABb	24.45 ± 0.40^Aa	1.48 ± 0.05^Ba	21.11 ± 1.67^Bc
	20-30	8.12 ± 0.00^Ae	1.03 ± 0.01^Cc	19.33 ± 0.19^Ba	20.39 ± 0.11^Cd	1.07 ± 0.12^Ca	10.97 ± 2.16^Ca

Fig. 2 Effects of different nitrogen additions on soil aggregate composition in a temperate grassland of Nei Mongol. Different lowercase letters indicate significant differences between different addition treatments, different uppercase letters indicate significant differences between different soil layers (p < 0.05). N0, N2, N5, N10 and N25 indicate nitrogen addition amounts of 0, 2, 5, 10, 25 g·m-2·a-1, respectively.

Table 2 Effects of different nitrogen additions on mean mass diameter (MMD) and fractal dimension (D) of soil aggregates in a temperate grassland of Nei Mongol (mean ± SE)

处理 Treatment	MMD			D
处理 Treatment	0-10 cm	10-20 cm	20-30 cm	0-10 cm	10-20 cm	20-30 cm
N0	1.453 ± 0.009^Cb	1.631 ± 0.005^Ae	1.548 ± 0.005^Bc	2.690 ± 0.002^Ab	2.597 ± 0.003^Ca	2.662 ± 0.001^Bb
N2	1.334 ± 0.004^Cd	1.673 ± 0.003^Ad	1.541 ± 0.000^Bc	2.677 ± 0.003^Ac	2.586 ± 0.002^Bb	2.675 ± 0.001^Aa
N5	1.670 ± 0.002^Ca	1.764 ± 0.005^Ab	1.689 ± 0.014^Bb	2.595 ± 0.002^Ad	2.500 ± 0.000^Be	2.592 ± 0.002^Ac
N10	1.168 ± 0.007^Ce	1.814 ± 0.003^Aa	1.684 ± 0.002^Bb	2.741 ± 0.001^Aa	2.516 ± 0.002^Cd	2.578 ± 0.002^Bd
N25	1.430 ± 0.007^Cc	1.691 ± 0.009^Bc	1.913 ± 0.001^Aa	2.674 ± 0.004^Ac	2.528 ± 0.002^Bc	2.518 ± 0.001^Ce

Fig. 3 Effects of different nitrogen additions on the activities of β-glucosidase (A), cellulose hydrolase (B), peroxidase (C) and polyphenol oxidase (D) in a temperate grassland of Nei Mongol (mean ± SE). Different lowercase letters indicate significant differences between different addition treatments; different uppercase letters indicate significant differences between different soil layers (p < 0.05). N0, N2, N5, N10 and N25 indicate nitrogen addition amounts of 0, 2, 5, 10, 25 g·m-2·a-1, respectively.

Fig. 4 Effects of different nitrogen additions on dissolved organic carbon (A), easily oxidized organic carbon (B), microbial biomass carbon (C) contents and geometric mean of active organic carbon (GMC) (D) in a temperate grassland of Nei Mongol (mean ± SE). Different lowercase letters indicate significant differences between different addition treatments, different uppercase letters indicate significant differences between different soil layers (p < 0.05). N0, N2, N5, N10 and N25 indicate nitrogen addition amounts of 0, 2, 5, 10, 25 g·m-2·a-1, respectively.

Fig. 5 Pearson’s correlations between soil active organic carbon and soil physicochemical properties, aggregate structure, extracellular enzyme activity in a temperate grassland of Nei Mongol. AGB, aboveground biomass; BG, β-glucosidase activity; BGB, belowground biomass; CB, cellulose hydrolase activity; D, fractal dimension; DOC, dissolved organic carbon content; EOC, easily oxidized organic carbon content; GMC, geometric mean of active organic carbon content; MBC, microbial biomass carbon content; MBN, microbial biomass nitrogen content; MMD, mean mass diameter; PER, peroxidase activity; POX, polyphenol oxidase activity; SD, soil density; SLD, soil layer depth; SWC, soil water content; TC, total carbon content; TN, total nitrogen content. *, p < 0.05.

Fig. 6 Structural equation model of soil active organic carbon fractions in a temperate grassland of Nei Mongol. The rectangular box indicates the variables contained in the model. The arrow indicates the direction of the assumed causal relationship. The number next to the arrow indicates the standardized path coefficient. The arrow width indicates the relationship strength. The solid arrow indicates a significant relationship (*, p < 0.05; **, p < 0.01). The dashed arrow indicates an insignificant relationship (p > 0.05), and R2 on the rectangular box indicates the variance ratio of the variable explained by its relationship with other variables. BGB, belowground biomass; DOC, dissolved organic carbon content; EOC, easily oxidized organic carbon content; HA, hydrolase activity; MBC, microbial biomass carbon content; MMD, mean mass diameter; NA, nitrogen addition; OA, oxidase activity; SD, soil density.

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