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

Authors: Mikha, Maysoon; MARAKE, MAKOALA - National University Of Lesotho

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2022
Publication Date: 8/20/2022
Citation: Mikha, M.M., Marake, M.V. 2022. Soil organic matter fractions and carbon distribution under different management in Lesotho, Southern Africa. Soil Science Society of America Journal. 81(1):140-155. https://doi.org/10.1002/saj2.20471.
DOI: https://doi.org/10.1002/saj2.20471

Interpretive Summary: Agricultural practices that increase soil organic matter (SOM) can enhance land sustainability, ecosystem functioning and soil carbon (C) storage. This study evaluated how SOM changed in response to two tillage practices (moldboard plow, MP and no-tillage, NT) and three rates of commercial fertilizer (limestone ammonium nitrate, LAN; 28-0-0) at 0, 100, and 200 kg N ha-1 (0, 89, and 175 lb N ac-1). We evaluated soil organic C (SOC), soil total nitrogen (STN), and several components of SOM, including particulate organic matter (POM) and mineral-associated organic matter (MAOM). The study site was established in 2008 in southern Africa. Soil samples were collected from four depth layers: 0-5, 5-10, 10-15, and 15-30 cm (0-2, 2-4, 4-6, and 6-12 inches). The SOC, STN, and POM mass decreased with depth in NT but increased with depth in MP within the tilled layer of 0-15 cm depth. The largest fraction of SOC was within fine-POM for MP (48%) and MOAM for NT (49%), both of which are susceptible for wind erosion. For both NT and MB, the C:N ratio was highest with coarse-POM (C:N ~ 25) and lowest with MAOM (C:N ~ 7.2); low C:N makes MAOM-C vulnerable for microbial decomposition. The 200-N fertilizer treatment increased SOC in the 0-15 cm depth by 36% for NT and 27% for MB compared with 100-N. The 200-N fertilizer treatment also increased POM by 23% for NT and 29% for MB compared with 100-N. This study shows distribution of SOC among the SOM fractions was partially influenced by land management. These promising findings point to the need for systematic efforts across cropland where NT could be the recommended management to increase SOC conservation, reduce SOM losses, and improve air quality, land sustainability, and human health.

Technical Abstract: Soil organic matter (SOM) is a complex mixture of multiple fractions with different soil organic carbon (SOC) content that can be influenced by management decisions. This study aimed at evaluating the effects of tillage practices (moldboard plow, MP and no-tillage, NT) and commercial fertilizer as limestone ammonium nitrate (LAN; 28-0-0) at three rates (0, 100, and 200 kg N ha-1) on soil organic C (SOC), soil total N (STN), and SOM fractions (particulate organic matter (POM); POM-C; and mineral-associated organic matter-C (MAOM-C). The study site was established in 2008 on the National University of Lesotho (NUL) Campus Farm in the Roma Valley of the Maseru District in Lesotho, located in southern Africa. The soil was classified as Berea series (mixed, mesic Plinthithaquic Dystrochrepts). Soil samples were collected from 0-5, 5-10, 10-15, and 15- 30 cm depth. At the 0-5, 5-10, 10-15 cm increments, SOC, STN, and POM mass decreased with depth in NT but increased with depth in MP. NT contained more SOC by 54%, STN by 47%, and POM by 40%, while MP contained higher SOC and STN by 17% and POM by 35% average across 0-15 cm compared with 15-30 cm depth. The SOC in the 0-15 cm depth increased by 27% with MP and by 36% with NT with 200-N compared with 100-N. The high N-rate (200 kg N ha-1) increased POM mass by 28.8% for MP and 22.6% for NT compared with 100-N. The benefit of MP from high N-rate was probably related to the low initial amount of POM, where small changes in POM will be pronounced, while the high amount of POM in NT could mask the effect of high N-rate addition. The coarse-POM mass accounted for 39% and fine-POM mass accounted for 61% of the changes that could occur in total POM mass. The high percentage of SOC was observed within fine-POM fraction (48.4%) in MP practice, which makes it susceptible to wind erosion, and within MAOM fraction (48.9%) in NT practice. The C:N ratio was the 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 unstable in soil and vulnerable for microbial decomposition. These findings suggested the need for coordinated efforts across cropland where conservation tillage and residue maintenance could be used to reduce SOC and POM losses and contribute to improved air quality, land sustainability, and human health.