Soil Organic Matter Alteration Velocity due to Land-Use Change: A Case Study under Conservation Agriculture

Abstract

The cultivation of native forest soils usually triggers a decline in soil organic matter (SOM) and a deterioration of aggregates. Although switching to conservation tillage (CT) can supply SOM, little is known about the temporal resolution of this change. This study aims to quantify changes in soil organic carbon (SOC) content and SOM composition of the same soil under 14 years of CT, plowing tillage (PT), and native forest (NF). Plowing ameliorates the macroaggregate-mediated loss in SOC content, in both the fine fraction and the coarse particles. Decades of CT can significantly increase both the microaggregates and fine particles related to SOC content, whereas in the finest fraction, the volume of recalcitrant SOC increased the most, and reached the original value under NF. Continuous plowing triggered SOM molecular size increases in both aggregates and the fine fraction, whereas switching to CT restored the molecular SOM size of the fine fraction only. Therefore, this fraction can be changed, even in short periods. Water dissolved the largest and middle-sized molecules of SOM, which are mainly from macroaggregates. Even if aggregation did not increase due to turning to CT, the content of the larger molecules of SOM increased in this short time.

Differences in Mineral Phase Associated Soil Organic Matter Composition due to Varying Tillage Intensity

“…In contrast to the traditional SOC dynamic models [48], the SOC content of the slow pools, such as the s+c-associated pool and rSOC, also decreased owing to cultivation. Then, the SOC content of these pools increased significantly because of reduced tillage during this short period, as reported by Rieder et al [18] ( Table 1). The highest SOC loss was in the occluded SOM within the aggregates, which decreased to 19% of the original content owing to PT.…”

“…The present study aimed to determine the SOM compositions of various SOM fractions in the same Luvisol in a native forest (NF) and under ploughing and conservation tillage (CT) based on FTIR spectroscopy. An additional goal was to compare the results with those obtained by water extractions from the same soil fractions provided by Rieder et al [18]. Our hypotheses were: (i) Shifts in tillage-triggered differences in the in situ SOM composition of the fractions; and (ii) these differences are not the same as those was provided by water extractions.…”

“…Three composite samples were collected from each land use type, including subsamples from upslope, middle slope, and downslope. A detailed description of the soil properties and tillage operations in the cultivated sites plus the forest site characteristics was presented by Rieder et al [18]. The climate is subhumid, with a mean annual temperature of 11 • C and precipitation of 628 mm.…”

“…This could have been the result of the highest tillage-induced SOM loss in this fraction. Rieder et al [18] presented OM composition results of the same soil fractions as those investigated in this study, and applied the dynamic light scattering method in water extractions. Their results did not show univocal tendencies, and suggested that ploughing triggered an increase in the dissolved OM molecular size in both the occluded and fine fraction-related OM.…”

“…In contrast, the composition of aggregate-occluded and mineral phase-associated SOM is not fully known. Previous studies have indicated that clay-associated SOM is generally composed of small molecular weight components and that the aggregate-occluded pool is generally composed of complex macromolecules [16][17][18]. As aromatic components are more resistant, their enrichment can refer to the increasing residence time of SOM [19] in the generalized order of free POM < occluded SOM < mineral-associated SOM [20].…”