The influence of land use on humic substances in three semi-arid agro-ecosystems in the Free State

[摘要] English: This study was initiated to complement earlier investigations into soil organicmatter degradation and restoration on account of agricultural land use in the FreeState Province of South Africa. In these studies no attention was given to theresponse of humic substances which represent the most active fraction oforganic matter. The aim with this study was therefore to quantify the influence ofagricultural land use on humic substances in soils of semi-arid regions.Topsoil (0-200 mm) samples from distinctive agro-ecosystems at Harrismith(Mean annual rainfall, MAR = 624 mm and Mean annual temperature, Ta =13.8°C), Tweespruit (MAR = 544 mm and Ta = 14.8°C) and Kroonstad (MAR =566 mm and Ta = 16.6°C) were selected for use in this study. An agroecosystemimplies a region where the three environmental factors affecting yield,namely climate, slope and soil are for practical purposes homogeneous. Theselected samples represent a virgin (grassland soil never cultivated before),cultivated (formerly grassland soil cultivated for at least 20 years) and restored(formerly cultivated soil converted to perennial pasture for at least 15 years)Plinthustalfs (10.6 to 13.5% clay) at every agro-ecosystem. Parametersquantified comprise crude soil, extractable soil, humic acid and fulvic acid Ccontents, N contents and C/N ratios. Concerning these parameters, cultivatedsoil was compared with virgin soil and restored soil with cultivated soil.The crude soil C content of the virgin soils varied from 7.3 g C kg-1 soil in thewarmer, drier Kroonstad agro-ecosystem to 21.6 g C kg-1 soil in the cooler, wetterHarrismith agro-ecosystem. Across agro-ecosystems the contribution ofextractable to crude soil C was almost constant, namely 47.1 to 48.4%. Thecontribution of humic acid C to extractable soil C decreased and that of fulvic acidC to extractable soil C increased from the Kroonstad to Harrismith agroecosystem.Cultivation reduced crude soil C in the three agro-ecosystems with 50.2 to51.8%. This is equivalent to absolute losses of 3.8, 8.2 and 10.8 g C kg-1 soil atKroonstad, Tweespruit and Harrismith agro-ecosystems respectively. Loss ofextractable soil C was more variable ranging from 36.7% or 1.3 g C kg-1 soil inthe warmer, drier Kroonstad agro-ecosystem to 48.2% or 5.1 g C kg-1 soil in thecooler, wetter Harrismith agro-ecosystem. Trends of this nature were nonexistentfor either humic or fulvic acid C losses.Gains of crude soil C ranged from 5.4 g C kg-1 soil in the warmer, drier Kroonstadagro-ecosystem to 8.0 g C kg-1 soil in the cooler, wetter Harrismith agroecosystem.This trend manifested also in extractable soil C gains which werelowest at Kroonstad (1.5 g C kg-1 soil) and highest at Harrismith (2.8 g C kg-1soil). Neither humic acid C nor fulvic acid C showed trends of this nature.The N contents although more variable than the C contents are to a large extentsupportive concerning organic matter in the virgin, cultivated and restored soils ofthe three agro-ecosystems. Further elaboration on the N contents is thereforenot justified here.Based on both C and N indices, it can be stated that humic substances did notshow explicit trends on account of land use as was the case with organic matterper se. This phenomenon warrants further investigation since humic substancesare regarded as the most reactive fraction of organic matter.

[发布机构] University of the Free State