Abstract
Cropland afforestation has been widely found to increase soil organic carbon (SOC) and soil total nitrogen (STN); however, the magnitudes of SOC and STN accumulation and regulating factors are less studied in dry, marginal lands, and therein the interaction between soil carbon and nitrogen is not well understood. We examined the changes in SOC and STN in younger (5–9-year-old) and older (25–30-year-old) black locust (Robinia pseudoacacia L., an N-fixing species) plantations that were established on former cropland along a precipitation gradient (380 to 650 mm) in the semi-arid Loess Plateau of China. The SOC and STN stocks of cropland and plantations increased linearly with precipitation increase, respectively, accompanying an increase in the plantation net primary productivity and the soil clay content along the increasing precipitation gradient. The SOC stock of cropland decreased in younger plantations and increased in older plantations after afforestation, and the amount of the initial loss of SOC during the younger plantations’ establishment increased with precipitation increasing. By contrast, the STN stock of cropland showed no decrease in the initial afforestation while tending to increase with plantation age, and the changes in STN were not related to precipitation. The changes in STN and SOC showed correlated and were precipitation-dependent following afforestation, displaying a higher relative gain of SOC to STN as precipitation decreased. Our results suggest that the afforestation of marginal cropland in Loess Plateau can have a significant effect on the accumulation of SOC and STN, and that precipitation has a significant effect on SOC accumulation but little effect on STN retention. The limitation effect of soil nitrogen on soil carbon accumulation is more limited in the drier area rather than in the wetter sites.
Highlights
Clearing trees to create cropland can reduce soil organic carbon (SOC) significantly by up to 50% of the SOC in the initial forest [1,2,3,4]
Our results suggest that the afforestation of marginal cropland in Loess Plateau can have a significant effect on the accumulation of SOC and soil total nitrogen (STN), and that precipitation has a significant effect on SOC accumulation but little effect on STN retention
Our study suggests that afforestation of marginal cropland could lead to a significant increase in SOC and STN stocks in a relatively short time and underlines the effect of precipitation on SOC dynamics during afforestation in an arid environment
Summary
Clearing trees to create cropland can reduce soil organic carbon (SOC) significantly by up to 50% of the SOC in the initial forest [1,2,3,4]. The afforestation of previously arable land is generally found to sequester carbon (C) in the soil and play an important role in climate change mitigation [5,6,7], which was proposed as an effective method of C sequestration in Article 3.3 of the Kyoto Protocol in 1997. Soil C sequestration following afforestation has been the subject of a substantial body of research, which suggests that the direction and magnitude of SOC changes are determined by many factors and processes, such as climate, stand age and soil depth [6,7,9,10,11]. Some reviews suggest that climate factors have a weak effect on soil C accumulation on a global scale [6,11], whereas other metaanalyses indicate that climate plays an important role in influencing SOC during afforestation [9,12]. It has been suggested that the different initial trend may depend on mean annual precipitation (MAP) [10], with soil C decreasing in wetter regions (MAP.1150 mm) but increasing in drier regions (MAP, 1150 mm)
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