Carbon sequestration potential and its main drivers in soils under alfalfa (Medicago sativa L.).

As a perennial forage crop, alfalfa (Medicago sativa L.) has been extensively utilized for the vegetation restoration of degraded soil and provides feedstock for forage. Its high usage can be attributed to its high yield potential and the increasing soil organic carbon (SOC) sequestration of alfalfa cultivation. However, the impact of land conversion to alfalfa on SOC content and its underlying drivers remain unclear. We performed a meta-analysis at the global scale to explore the quantified effects of alfalfa cultivation on SOC content and identify its controlling factors. We employed 1699 pairwise data points from 90 publications based on cropland/abandoned land conversion to alfalfa. Globally, cropland (cropland-alfalfa) and abandoned land (abandoned land-alfalfa) conversion to alfalfa enhanced SOC content by 12.1 % and 13.7 %, respectively. Alfalfa exhibited greater SOC content benefits in the surface soils (0-20 cm) with a lower level of initial SOC (<16 g kg-1), regardless of the land conversion type. Cropland-alfalfa was observed to increase SOC content with fertilization, irrigation, and conventional tillage in the long term (>5 years). Furthermore, abandoned land-alfalfa enhanced SOC content in the absence of alfalfa biomass removal and for longer cultivation durations (>5 years). Boosted regression tree analyses indicated variations in soil properties (75 % for cropland-alfalfa and 65 % for abandoned land-alfalfa) as the primary factors driving changes in SOC content. The dominant drivers were determined as the soil layer (51.6 %), cultivation duration (13.1 %), and initial SOC (12.9 %) for cropland-alfalfa, and initial SOC (43.7 %), soil layer (24.6 %) and cultivation duration (17.1 %) for abandoned land-alfalfa. Land conversion to alfalfa has great potential for SOC sequestration, particularly in low-fertility soils. Therefore, alfalfa cultivation is highly recommended for degraded lands due to its SOC sequestration benefits in vegetation restoration.

Effect of Precipitation Variation on Soil Respiration in Rain-Fed Winter Wheat Systems on the Loess Plateau, China.

Global climate change has aggravated the hydrological cycle by changing both the amount and distribution of precipitation, and this is especially notable in the semiarid Loess Plateau. How these precipitation variations have affected soil carbon (C) emission by the agroecosystems is still unclear. Here, to evaluate the effects of precipitation variation on soil respiration (Rs), a field experiment (from 2019 to 2020) was conducted with 3 levels of manipulation, including ambient precipitation (CK), 30% decreased precipitation (P−30), and 30% increased precipitation (P+30) in rain-fed winter wheat (Triticum aestivum L.) agroecosystems on the Loess Plateau, China. The results showed that the average Rs in P−30 treatment was significantly higher than those in the CK and P+30 treatments (p < 0.05), and the cumulative CO2 emissions were 406.37, 372.58 and 383.59 g C m−2, respectively. Seasonal responses of Rs to the soil volumetric moisture content (VWC) were affected by the different precipitation treatments. Rs was quadratically correlated with the VWC in the CK and P+30 treatments, and the threshold of the optimal VWC for Rs was approximately 16.06−17.07%. However, Rs was a piecewise linear function of the VWC in the P−30 treatment. The synergism of soil temperature (Ts) and VWC can better explain the variation in soil respiration in the CK and P−30 treatments. However, an increase in precipitation led to the decoupling of the Rs responses to Ts. The temperature sensitivity of respiration (Q10) varied with precipitation variation. Q10 was positive correlated with seasonal Ts in the CK and P+30 treatments, but exhibited a negative polynomial correlation with seasonal Ts in the P−30 treatment. Rs also exhibited diurnal clockwise hysteresis loops with Ts in the three precipitation treatments, and the seasonal dynamics of the diurnal lag time were significantly negatively correlated with the VWC. Our study highlighted that understanding the synergistic and decoupled responses of Rs and Q10 to Ts and VWC and the threshold of the change in response to the VWC under precipitation variation scenarios can benefit the prediction of future C balances in agroecosystems in semiarid regions under climate change.