Deep soil water depletion and soil organic carbon and total nitrogen accumulation in a long‐term alfalfa pasture

Abstract

AbstractDeep 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−1 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−2, 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.