No abstract

Deep soil resources are essential for improving the production and ecological functions of dryland ecosystems. Relationships between deep soil water depletion and carbon and nitrogen accumulation in long-term dryland pastures were not well quantified and understood. This study aimed to quantify the changes in deep soil water, soil organic carbon (SOC), and total nitrogen (TN) in alfalfa pasture and identify their relationships with root density. We conducted a field experiment in the south Loess Plateau in 2020 and 2021. Soil water, SOC, and TN contents and alfalfa root length density to 1000-cm-depth were measured in alfalfa pastures and annual crop reference fields.Soil water depletion by alfalfa mainly occurred in the first 6 years with a depletion rate of 41.6, 49.1, and 62.1 mm yr À1 in the shallow (0-200 cm), middle (200-500 cm) and deep (500-1000 cm) soil layers, respectively. Total depletion after 6 and 19 years were 916.8 and 1049.4 mm, respectively. SOC and TN storages in the shallow and middle layers continuously increased and peaked at 19 years after planting, and those in the deep soil increased quickly in the early ages but showed few changes in the following years. Final SOC and TN storages in the 0-1000 cm profile in alfalfa pasture reached 61.1 and 5.4 kg m À2 , respectively, 13.7% and 20.4% higher than the reference field.Soil in 200-1000 cm contributed to 60.7% and 47.8% of the total SOC and TN storage increments, respectively. The deep rooting system of alfalfa derived soil water depletion and SOC and TN increments, as indicated by the significant relationships between fine root length density and the changes in soil water, SOC and TN storages. Therefore, the deep soil contributed substantially to the carbon and nitrogen sequestration in alfalfa early ages but the contribution was limited by water shortage in the middle and late ages. We suggest stopping alfalfa early (≤6 years) to increase water sustainability and maintain carbon and nitrogen sequestration efficiencies in the alfalfa pasture and crop rotation system on Loess Plateau.

Incorporating perennial pastures into annual crop systems is an efficient means of improving soil carbon (C) sequestration and reducing the application of nitrogen (N) fertilizer on farmlands. How the soil C and N at different soil depths respond to the length of pasture duration and rainfall conditions is still being determined. In this study, we conducted a meta-analysis of data from 63 published studies to investigate the impacts of the alfalfa pasture on the incorporation of soil organic carbon (SOC), total nitrogen (STN), and available nitrogen (SAN) contents in the 0–300 cm soil profile of the Loess Plateau. An annual crop field was taken as a reference. The results showed that the average SOC content at soil depths of 0–100 and 100–200 cm in the alfalfa pasture increased by 17% and 8% (p < 0.001) compared to the crop field, respectively, while that at 200–300 cm decreased (p > 0.05). The SOC content increased with pasture age; it was the highest when the alfalfa had been planted for 5–9 years and decreased thereafter. The STN content at soil depths of 0–100 and 100–200 cm increased by 19% and 14% (p < 0.001), respectively; the content at depths of 200–300 cm only increased slightly (p > 0.05). It also increased the most when the alfalfa was 5–9 years old. The increments in the SAN content at the 0–100 and 100–200 cm soil depths were higher than those of the STN, with values of 29% and 18%, respectively, while those at depths of 200–300 cm also changed insignificantly (p > 0.05). The SAN content continuously increased with the age of the alfalfa, and the average increment in the 0–300 cm profile was as high as 21% when the alfalfa was ≥10 years old. The SOC and STN content increased the most under moderate rainfall conditions (350–500 mm), while the SAN content maintained the highest increment under high rainfall (500–650 mm) conditions. Therefore, ley farming with the alfalfa pasture contributed substantially to the soil C and N at depths of 200 cm in deep loess. Alfalfa should be removed in its middle ages to increase C sequestration while utilizing soil N efficiently.