Soil carbon and nitrogen changes following afforestation of marginal cropland across a precipitation gradient in Loess Plateau of China

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.

Figure 1. Comparison of fine root biomass among cropland and younger and older plantations in each site as well as among the younger and older plantations in four sites.

Younger plantations are 5- to 9-year- old and older plantations are 25- to 30- year-old forests, respectively. The error bars represent the standard deviations of the means. The sample numbers are shown in Table 1. A different lowercase English letter is used for cropland and for younger and older plantations in the same site, and a different uppercase English letter represents younger plantations across the sites, whereas a different Greek letter is used for older plantations to indicate a significant difference at the 0.05 level.

Figure 2. Relationship between mean annual precipitation and soil organic carbon (SOC), soil total nitrogen (STN) stock, and C:N ratio in cropland and younger and older plantations.

Younger and older plantations are 5- to 9-year-old and 25- to 30-year-old forests, respectively. The error bars represent the standard deviations of the means (only shown in positive, n = three to six for each stand). The data for some cropland stands were cited from other studies (Table 1). Regressions were conducted for the top 20 cm of soil, and only significant regression models are displayed.

Figure 3. Relationship between mean annual precipitation and the absolute and relative change in soil organic carbon (SOC) in younger and older plantations.

Younger and older plantations are 5- to 9-year-old and 25- to 30-year-old forests, respectively. Regressions were conducted separately for different soil layers, and only significant regression models are displayed. Positive values indicate an increase in the SOC stock due to afforestation, and negative values indicate a decrease.

Figure 4. Association of absolute and relative change in soil total nitrogen (STN) in younger and older plantations with mean annual precipitation.

Younger and older plantations mean 5- to 9-year-old and 25- to 30-year-old forests, respectively. Positive values indicate an increase in the STN stock due to afforestation, and negative values indicate a decrease. No significant association was found.

Figure 5. Relationship of changes in absolute soil organic carbon (SOC) and soil total nitrogen (STN) (A), changes in relative SOC and STN (B), and changes in the C:N ratio in younger (C) and older (D) plantations with mean annual precipitation.

Younger and older plantations mean 5- to 9-year-old and 25- to 30-year-old forests, respectively. The dashed line in panel B indicates the 1∶1 line. Values greater than 1.0 in panels C and D indicate a relative gain of SOC to STN following afforestation, and lower values indicate the inverse.