Responses of soil nitrogen in different soil organic matter fractions to long-term nitrogen addition in a semi-arid grassland

doi:10.17521/cjpe.2021.0044

Abstract

Abstract:

Aims Soil nitrogen (N) plays a vital role in regulating the structure and function of ecosystems and is affected by N deposition. Most previous studies focus on the responses of the N content in bulk soil to N deposition, but the responses of the N content in different soil organic matter (SOM) fractions remain unclear. We aimed to investigate how long-term N addition influenced soil N of different SOM fractions in a semi-arid grassland.

Methods A manipulated N addition experiment with 4 levels of N addition (0, 8, 32 and 64 g·m^-2·a^-1) has been conducted for 13 years in Duolun country, Nei Mongol. SOM was separated to particulate organic matter (POM) and mineral associated organic matter (MAOM) by density fractionation. The plant and soil properties were also measured.

Important findings The results showed that N addition had no significant effect on the carbon (C) content in bulk soil, POM, or MAOM. With increasing levels of N addition, the N content in bulk soil and in POM increased significantly. Furthermore, we found that the increased N content of POM was mainly associated with greater aboveground biomass following N addition. The N content of MAOM is mainly correlated with soil texture, but was not affected by N addition. These results suggest that continuous N addition can increase the soil N in bulk soil, but the increased N is mostly distributed in labile POM pools, which can be vulnerable to land use and climate change.

Key words: nitrogen addition, nitrogen content, particulate organic matter, mineral associated organic matter

WU Yun-Tao, YANG Sen, WANG Xin, HUANG Jun-Sheng, WANG Bin, LIU Wei-Xing, LIU Ling-Li. Responses of soil nitrogen in different soil organic matter fractions to long-term nitrogen addition in a semi-arid grassland[J]. Chin J Plant Ecol, 2021, 45(7): 790-798.

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https://www.plant-ecology.com/EN/Y2021/V45/I7/790

Figures/Tables 8

Fig. 1 Effects of nitrogen (N) addition on aboveground biomass (AGB)(A), dissolved inorganic nitrogen (DIN) content (B), soil pH (C) and microbial biomass carbon (MBC) content (D)(mean ± SE). F and p are the results of one-way ANOVA. Different lowercase letters indicate significant difference among N addition levels (p < 0.05).

Fig. 2 Effects of nitrogen (N) addition on soil clay and silt (A) and sand (B) content (mean ± SE). F and p are the results of one-way ANOVA. Different lowercase letters indicate significant difference (p < 0.05) among different N addition levels.

Fig. 3 Effects of nitrogen (N) addition on soil carbon (C) content (A), soil N content (B) and soil C:N (C)(mean ± SE). F and p are the results of one-way ANOVA. Different lowercase letters indicate significant difference (p < 0.05) among different N addition levels.

Fig. 4 Effect of nitrogen (N) addition on the relative mass (A), carbon (C) content (B) and N content (C) of particulate organic matter (POM) and mineral associated organic matter (MAOM)(mean ± SE). F and p are the results of one-way ANOVA. Different lowercase letters indicate significant difference (p < 0.05) among different N addition levels.

Fig. 5 Relationships between the soil carbon (C) in different SOM fractions and nitrogen (N) addition rates (A), and soil clay and silt content (B); and relationships between the soil N in different SOM fractions and N addition rates (C), and soil clay and silt content (D). Solid and dashed lines represent significant (p < 0.05) and insignificant (p > 0.05) relationships, respectively. □, mineral associated organic matter (MAOM); △, particulate organic matter (POM).

Table 1 Summary of the generalized linear mixed model (GLMM) for the effects of nitrogen (N) addition and silt and clay content on the carbon (C) and N content in particulate organic matter (POM) and mineral associated organic matter (MAOM)

响应变量 Response variable	变量 Variable	相关系数 Correlation coefficient	标准误 Standard error	t	p
POM碳含量 C content in POM	氮添加水平 N addition (g·m^-2·a^-1)	0.022	0.028	0.793	0.446
POM碳含量 C content in POM	黏粒与粉粒含量 Silt and sand clay content (%)	0.013	0.187	-0.068	0.947
MAOM碳含量 C content in MAOM	氮添加水平 N addition (g·m^-2·a^-1)	-0.043	0.019	-2.287	0.045
MAOM碳含量 C content in MAOM	黏粒与粉粒含量 Silt and sand clay content (%)	0.396	0.126	3.145	0.010
POM氮含量 N content in POM	氮添加水平 N addition (g·m^-2·a^-1)	0.014	0.003	4.045	0.002
POM氮含量 N content in POM	黏粒与粉粒含量 Silt and sand clay content (%)	-0.030	0.021	-2.086	0.182
MAOM氮含量 N content in MAOM	氮添加水平 N addition (g·m^-2·a^-1)	-0.002	0.001	-1.408	0.162
MAOM氮含量 N content in MAOM	黏粒与粉粒含量 Silt and sand clay content (%)	0.035	0.010	3.586	0.005

Table 2 Summary of the best corrected Akaike Information Criterion (AICc)-selected models for the nitrogen (N) content in particulate organic matter (POM) and mineral associated organic matter (MAOM)

响应变量 Response variable	因子 Factor	模型Model
响应变量 Response variable	因子 Factor	1	2	3	4
POM氮含量 N content in POM	常数项 Constant term	1.770	1.697	0.090	-
	地上生物量 Aboveground biomass	0.003	0.002	0.003	-
	微生物生物量碳含量 Microbial biomass carbon content	0.002	-	-	-
	C:N	-1.111	-0.067	-	-
	赤池信息准则 Akaike information criterion	24.90	21.95	17.66	-
MAOM氮含量 N content in MAOM	常数项 Constant term	1.202	1.451	0.762	0.702
	黏粒与粉粒含量 Silt and clay content	0.031	0.029	0.036	0.030
	微生物生物量碳含量 Microbial biomass carbon content	-0.002	-0.001	-	-
	异养呼吸速率 Heterotrophic respiration rate	0.375	-	-	-
	地上生物量 Aboveground biomass	-0.002	-0.002	-0.001	-
	赤池信息准则 Akaike information criterion	5.92	5.17	3.01	2.30

Fig. 6 A schematic representation of the structural equation modeling (SEM) analysis used to identify the controls of soil nitrogen (N) content in particulate organic matter (POM)(A) and mineral associated organic matter (MAOM)(B) under N addition. Results of the model fitting were: POMN, X2 = 0.480, p = 0.787, comparative fit index (CFI) = 1.000, root-mean-square (RMSEA) < 0.001; MAOMN, X2 = 2.856, p = 0.414, CFI = 1.000, RMSEA < 0.001. Solid and dashed arrows represent significant effect (p < 0.05) and insignificant effect (p > 0.05), respectively. Number adjacent to the arrows refer to the standardized path coefficients. AGB, aboveground biomass; DIN, dissolved inorganic nitrogen; MBC, microbial biomass carbon.

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