Phosphorus legacy: role of long-term soil phosphorus accumulation in the sustainable management of intensive agroecosystems

Abstract

Phosphorus (P) is an essential nutrient for all organisms. Phosphate rock is primarily utilised for the manufacture of P fertilisers, and is a finite resource. Most agricultural lands worldwide present low levels of available soil P, thus requiring P inputs for productive agriculture. However, constant P inputs result in accumulation of soil P (legacy P), increasing risk of eutrophication of waterways. Efficient P use of agriculture require improvements in utilisation of legacy P. The objective of this work was to investigate and quantify the impact of contrasting agricultural land uses and management on the nature and dynamics of P. Three long-term, replicated field trials were selected: i) long-term irrigation trial, Winchmore, New Zealand; ii) long-term pig slurry inputs, Santa Catarina, Brazil; iii) long-term ecology trial, Lincoln, New Zealand. Four experiments were conducted. The first study investigated the impact of 62 years of irrigation on the amounts and distribution of soil profile P to 100 cm under grazed pasture. Despite identical P inputs, total soil profile P accumulation was inversely proportional to water input rates (6423, 5908 and 5054 kg P ha⁻¹ for the control, low and high irrigation rates, respectively). Differences were mainly attributed to inorganic P forms. Phosphorus removal and transfer/loss occurred under irrigation. For a 3-fold increase in irrigation frequency, P removal in irrigation outwash increased by 13-fold. Combined, annual removal in animal products, internal transfer, and outwash losses were directly related to irrigation frequency and increased from 8 to 18.6 kg P ha⁻¹ for treatments receiving annually, 2.6 or 7.7 100-mm irrigations, respectively. The second experiment quantified the impacts of P inputs in pig slurry to a high P-sorbing Oxisol under cropping in southern Brazil. Fifteen years of slurry additions resulted in P accumulations and vertical movement proportional to application rates. However, changes were confined to the 0-20 cm depth. Phosphorus accumulated mainly in inorganic forms. Slurry input rates of 25, 50, 100 and 200 m³ ha⁻¹ y⁻¹ resulted in accumulations of 25, 57, 106 and 159 kg P ha⁻¹ y⁻¹ (0-40 cm), where 8, 10, 23 and 28 kg P ha⁻¹ y⁻¹ were organic P forms. Mass balance confirmed that most of P added to the system accumulated in the soil. The third and fourth experiments assessed, respectively, long-term and short-term impacts of plant biomass retention or removal in soil biogeochemical properties after 20 years, in absence of P inputs. Grassland plants utilised 35% of the P legacy, mainly from inorganic forms. Plant production and P uptake were up to 2-fold higher for the biomass retained comparatively to biomass removed. Mineralisation of soil organic P was limited following P depletion. Contrastingly, despite increased microbial P immobilisation soils under biomass retained, 20% faster turnover rates and 2-fold increase in P fluxes through microbial biomass were observed. The collective findings of this research show that legacy P in soils plays a dominant role in determining P availability as influenced by land management. Further research is necessary to investigate strategies to enhance legacy P mobilisation and utilisation by plants.