Soil organic matter fractions and carbon distribution under different management in Lesotho, southern Africa

Soil organic matter (SOM) is a complex mixture of multiple fractions of soil organic C (SOC) that can be influenced by management decisions. This study evaluate the effects of tillage (moldboard plow [MP] and no-tillage [NT]) and commercial fertilizer as limestone ammonium nitrate (28–0–0) at three rates (0 [0-N], 100 [100-N], and 200 [200-N] kg N ha^–1) on SOC, soil total N (STN), and SOM fractions (total particulate organic matter-C [POM-C] and mineral-associated organic matter-C [MAOM-C]). The study was established in 2008 on the National University of Lesotho Campus Farm, Roma Valley of the Maseru District in Lesotho, southern Africa. The soil classified as Berea series. Soil samples were collected from 0-to-5-, 5-to-10-, 10-to-15-, and 15-to-30-cm depths. Under NT, the SOC, STN, and POM at 0-to-15-cm were 54, 47, and 40% higher than 15-to-30-cm depth, respectively. Differently, 0-to-15-cm the MP had 17, 17, 35% higher SOC, STN, and POM than 15-to-30-cm depth. The SOC in the 0-to-15-cm increased by 27% with MP and by 36% with NT at the 200-N compared with the 100-N rate. The highest N-rate (200-N) increased total POM by 28.8% for MP and 22.6% for NT than the100-N rate. The greater increase in POM under MP with high N rate was probably related to the low initial total POM, where small changes in POM will be pronounced, whereas the high total POM in NT could mask the effect the high N rate addition. The coarse and fine POM accounted for 39 and 61%, respectively, of the changes in total POM at both NT and MP. The high percentage of SOC was observed within fine POM fraction (48.4%) in MP, which makes it susceptible to wind erosion, and within MAOM fraction (48.9%) in NT practice. The C/N ratio was highest with coarse POM (C/N ≈ 25) and lowest with MAOM (C/N ≈ 7.2) at both tillage practices. The low C/N ratio made MAOMC vulnerable to microbial decomposition. These findings suggest the need for conservation efforts where conservation tillage and N rates could be used to reduce SOC and POM losses and contribute to improve land sustainability and SOM conservation.