Land use effects on subtropical, sandy soil under sandyzation/desertification processes

Abstract

Very fragile sandy soils have high erosion susceptibility, low water retention, and low nutrient and carbon storage. In southern Brazil, substitution of native grassland by grain production in conventional tillage has caused severe degradation, even reaching an extreme state of “sandyzation” or non-vegetated sand. We measured indicators to assess soil quality in a Quartzipsamment (“Neossolo Quartzarênico”) under different uses and also in extreme state of “sandyzation” or non-vegetated sand, where annual average rainfall is 1511mm with regular distribution throughout the year. Four land uses/conditions were evaluated: (i) native grassland (NG) (4% clay); (ii) nine years-old eucalyptus forest (EF) (4% clay); (iii) corn production in conventional tillage (CT) (3% clay); and (iv) extreme state of “sandyzation” or non-vegetated sand (NVS) (2% clay). Measured soil chemical properties were soil organic carbon (SOC) in 0–2.5, 2.5–5, 5–7.5, 7.5–10 and 10–20cm soil layers, Ca, Mg, K, Al saturation, and effective cation exchange capacity (CEC) in 0–2.5cm layer. Measured soil physical properties were soil bulk density (BD), total porosity (TP), macroporosity (Macro), microporosity (Micro), water retention, and saturated hydraulic conductivity (Ks) in 0–3, 5–8, and 10–13cm soil layers, whereas aggregate stability in water was measured in 0–5cm soil layer. Soil chemical and physical properties indicate an extreme state of degradation of NVS, which had significantly lowest SOC and thus low CEC, associated to leaching of exchangeable bases and high aluminum saturation. CT also provided a significant decrease in SOC in all soil layers, and other in chemical properties in the 0–2.5cm layer, whilst EF was the most efficient system to build up SOC compared to NG in surface layers (0–2.5 and 2.5–5cm) and presented better chemical conditions. In general, soil physical quality was degraded in NVS and improved in EF, whilst there were no significant differences in CT compared to NG for most soil physical properties. Soil physical properties were closely related to surface SOC of the different land uses. NVS provided a significant decrease in Micro, whilst EF provided a significant increase in TP and Macro in 0–3 and 5–8cm soil layers. High TP and Macro, low Micro, and very high Ks were observed in NVS. The CT provided significant decrease in water aggregate stability compared to NG and the water aggregate stability rank was EF>NG>CT, whereas NVS soil was completely devoid of aggregation. NVS restricts plant growth, root development, plant nutrient uptake and soil cover, thus creating an environment prone to wind erosion and soil degradation. Management practices that include permanent soil cover, restore SOC, improve soil aggregation and create pores for water retention and availability, increase base saturation and promote nutrient cycling are necessary to preserve these fragile lands.