Towards an integrated phosphorus, carbon and nitrogen cycling model for topographically diverse grasslands

Abstract

Contemporary science on how livestock influence nutrient cycling in grazing systems is limited, particularly in topographically complex (i.e., slopes and aspects) hill country landscapes. Prominent slope and aspect variation affects primary production, animal behaviour and nutrient return. Here, we embed recent scientific advancements in nutrient dynamics across complex landscapes to (1) set up a soil organic carbon (SOC) saturation function to an existing SOC and total soil phosphorus (TSP) model (Bilotto et al. J N Z Grassl 81:171–178, 2019. https://doi.org/10.33584/jnzg.2019.81.397), (2) include total soil nitrogen (TSN) dynamics, and (3) establish if the model (herein the Grass-NEXT model) can simulate the spatial and temporal changes of TSP, SOC and TSN in hill country. A long-term P fertiliser experiment with contrasting different P fertilisation levels and associated sheep stocking regimes (herein, ‘farmlets’) was used for model testing. The Grass-NEXT model predicted TSP and SOC stocks with strong accuracy and precision (model performance), and TSN with a moderate performance across farmlets [Concordance Correlation Coefficient (CCC), 0.75, 0.72 and 0.49, respectively]. Grass-NEXT model simulated TSP, SOC and TSN distribution with moderate/strong performance across slopes (CCC, 0.94, 0.80 and 0.70) and aspects (CCC, 0.83, 0.67 and 0.51). Consistent with observed data, modelled changes in TSP and TSN were greater on low slopes and eastern aspects, but no clear pattern was observed for SOC stocks. The Grass-NEXT model provides an intuitive research tool for exploring management options for increasing SOC and TSN, as well as an instrument for monitoring and reporting on nutrient dynamics in complex landscapes.

In recognition of Mike Hedley: fate of fertiliser in soil and mobilisation of recalcitrant nutrients

“…All the papers are focused on P and have often used the P fractionation technique. However, much like the diversity of topics covered over Mike's career, these papers cover topics such as: minimising P losses from fertiliser to water (Wang et al 2022); improving the efficiency of P use in grassland and cropping systems (Bilotto et al 2022;Bouray et al 2022;Hu et al 2022;Schwerdtner et al 2022;Touhami et al 2022), assessing the effect of different amendments on the availability of P in the soil (Brownrigg et al 2022;Kruse et al 2022); and fertiliser value of waste streams (Battisti et al 2022).…”

Soil organic nitrogen variation shaped by diverse agroecosystems in a typical karst area: evidence from isotopic geochemistry

“…Hence, the δ 15 N SON composition of the soil environment may be strongly governed by agricultural activities. Previous studies in the field of N cycling have mainly focused on the storage of individual forms of N (such as NH 4 + -N and NO 3 – -N) or the distribution of soil total N, as well as the shallow soil of a specific type during long-term monitoring ( Bilotto et al, 2022 ; Xiao et al, 2018 ). Comparative analyses of SON variation and δ 15 N SON composition are limited in whole soil profiles under different land utilizations, especially in karst areas.…”