Purpose

Different growth stages of vegetation are vital factors controlling soil organic carbon (SOC) sequestration on the tableland of the Loess Plateau. To ascertain the SOC sequestration process and direction, we studied the changes in SOC fractions and the relationships with enzyme activities in different stages of restoration with different vegetation types.

Materials and methods

Soils from different stages of farmland conversion into grassland or forestland were selected, in which the space sequence and sampling time were combined to replace time series. SOC, particulate OC (POC), light fraction OC (LFOC), dissolved OC (DOC), and soil enzyme activities of catalase, urease, alkali-phosphatase, and inverse were measured. The change process of labile organic carbon (LOC) was performed to study the direction of SOC in the succession of different vegetation types. The effect of SOC–soil enzyme activity interactions was evaluated by RDA variance decomposition to illustrate the role of soil enzyme activity in SOC sequestration.

Results and discussion

With the extension of the planting time, the SOC fractions and total nitrogen under Hippophae rhamnoides (HR) increased rapidly from 0 to 16 and 16 to 21 years and decreased rapidly from 21 to 30 years, while LOC showed greater variation. LOC under Pinus tabulaeformis (PT) changed more slowly than that under HR. The sensitivity of different LOC to land use varied. From the change rate of soil LOC, we concluded the DOC changed the fastest, followed by the LFOC, the CPOC in POC was the slowest in the extension of vegetation growth time. By RDA variance decomposition, fine POC was mainly affected by urease; coarse POC in POC was mainly affected by alkaline phosphatase, which was promoted by invertase; LFOC was mainly promoted by catalase; DOC was mainly affected by alkaline phosphatase. Therefore, during the progression of SOC change, the initial stage was obviously affected by alkaline phosphatase. Catalase played a leading role during the gradual decrease in SOC activity. When SOC became stable, it was instead mainly affected by urease.

Conclusions

Land use changes and different growth stages of vegetation altered the SOC sequestration process and direction. The long-term return of farmland to grassland would greatly promote the concentrations of SOC fractions, and the short-term return of farmland to PT or HR would also encourage an increase in these fractions, while the long-term would have the opposite effect. Therefore, in order to effectively improve SOC concentrations, during vegetation restoration, it should be properly matched in vegetation species and stages.

中文翻译：

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黄土高原地区不同植被类型演替过程中土壤有机碳组分的变化趋势

目的

植被的不同生长阶段是控制黄土高原旱地土壤有机碳固存的重要因素。为了确定SOC的固存过程和方向，我们研究了不同植被类型的不同恢复阶段SOC组分的变化以及与酶活性的关系。

材料和方法

选择农田转化为草地或林地不同阶段的土壤，并结合空间序列和采样时间来代替时间序列。测量了SOC，微粒OC（POC），轻度OC（LFOC），溶解OC（DOC）以及过氧化氢酶，脲酶，碱性磷酸酶和逆酶的土壤酶活性。进行了不稳定有机碳（LOC）的变化过程，以研究SOC在不同植被类型中的变化方向。通过RDA方差分解评估SOC-土壤酶活性相互作用的影响，以说明土壤酶活性在SOC螯合中的作用。

结果和讨论

随着播种时间的延长，沙棘（HR）下的SOC分数和总氮从0年迅速增加到16年，从16年增加到21年，而从21年减少到30年迅速，而LOC表现出更大的变化。油松下的LOC（PT）的变化比HR下的变化要慢。不同LOC对土地利用的敏感性各不相同。从土壤LOC的变化率来看，结论是DOC变化最快，其次是LFOC，POC中的CPOC在植被生长时间延长中最慢。通过RDA方差分解，精细POC主要受脲酶的影响；POC中的粗POC主要受碱性磷酸酶的影响，而碱性磷酸酶受转化酶的促进。LFOC主要由过氧化氢酶促进。DOC主要受碱性磷酸酶的影响。因此，在SOC变化的过程中，初始阶段明显受到碱性磷酸酶的影响。过氧化氢酶在SOC活性逐渐降低的过程中起主导作用。当SOC稳定后，它主要受脲酶的影响。

结论

土地利用的变化和植被的不同生长阶段改变了SOC的固存过程和方向。长期将农田归还给草地将大大提高SOC组分的浓度，而短期将农田归还给PT或HR也将鼓励增加这些组分，而长期的作用则相反。因此，为了有效地提高土壤有机碳的浓度，在植被恢复过程中，应在植被物种和阶段进行适当的匹配。