Benchmark sites for assessing the chemical impacts of pastoral farming on loessial soils in southern New Zealand

Abstract

Chemical differences between soils under farmland and under reserve vegetation were compared at nine sites on Pallic, Brown and Melanic soils (Fragiaquepts, Dystrochrepts and Haplumbrepts) in Otago and Southland, New Zealand. Farmland topsoils were more compact than topsoils of reserves, were less acid, and contained more total Al, Si, Fe, Ca, K, P and Cd. The greater content of total Al, Si, Fe and K was explained by the lower total C content and the higher bulk density of farmland soils. Higher concentrations of Ca, P, and Cd in farmland soils were explained by lime and superphosphate additions. Calcium equivalent to that contained in about 8 t/ha of lime and P equivalent to that contained in 4.5–6.5 t/ha of superphosphate have accumulated in the farmland topsoils. Farmland topsoils contained 3 to 4 times more total Cd than reserve soils because of Cd added in superphosphate, but the highest soil Cd levels were only a fraction of the maximum level recommended by the New Zealand Department of Health. Despite the ubiquitous legumes on farmland soils, farmland soils did not contain more N than reserve soils, indicating that N gain from fixation of about 140 kg ha −1/year is probably balanced by the N loss from leaching and volatilisation. Farmland subsoils were less acid to 1 m depth, and had higher pH and exchangeable Ca and total P concentrations than reserve soils. These differences were attributed to penetration of Ca (from lime) and P (from superphosphate) into subsoils. The P increase was greatest in Brown soils, which at 0–60 cm depth had accumulated P equivalent to that contained in about 17 t/ha of single superphosphate. The study shows that reserves are valuable as soil benchmark sites, particularly for measuring the extent of pH, soil compaction and C and N change produced by farming practices, and the fate of lime, P and contaminants applied to soils.