# Multiyear changes in snowmelt phosphorus runoff with soil P drawdown or application of struvite to an organic forage crop

## TL;DR

This study shows that using struvite fertilizer or harvesting perennial hay can reduce phosphorus runoff in snowmelt, helping protect water quality while managing soil fertility.

## Contribution

The study compares long-term effects of struvite application and soil P drawdown on reducing phosphorus runoff in organic forage systems.

## Key findings

- Phosphorus runoff decreased over time with repeated harvests of perennial hay crops.
- Struvite application resulted in lower phosphorus runoff than predicted by regional models.
- Soil phosphorus levels strongly influence the risk of phosphorus loss in snowmelt.

## Abstract

In this study, changes in P runoff losses from three field‐scale small watersheds in the Northern Great Plains region (Manitoba, Canada) under organic management were evaluated over a 7‐year period (2017–2023). A perennial forage crop was grown through this time period, either with biennial addition of a slow‐release fertilizer to maintain soil fertility (struvite, 4 site‐years) or without P amendment (drawdown, 8 site‐years). Annually measured soil P (Olsen P) strongly correlated with mass balance of P added as struvite or removed by crops and surface runoff. As a result, P limitation of forage yield was observed in the later years of the drawdown treatment. P loads and flow‐weighted mean concentrations (FWMCs) measured in snowmelt runoff after struvite application were compared to predicted values using published regional models developed with 82 site‐years of data from field‐scale small watersheds, with Olsen P, water yield, and percent surface cover as predictors. To evaluate response to struvite over a wider range of conditions, P runoff was measured in two additional watersheds under conventional annual grain production following struvite application (2 site‐years). Higher risk of P loss was observed with higher Olsen P. However, following struvite applications, the risk of P loss to snowmelt was consistently lower than predicted (between −0.13 and −0.85 mg L−1 for FWMC of total dissolved P) suggesting that the solubility of struvite in runoff water may be lower than for other forms of soil P also extractable as Olsen P.

Drawdown of surface soil P with repeated harvest of perennial hay reduces potential for snowmelt runoff losses.Struvite added to soils significantly increased 0–5 cm Olsen P (147 ± 85%).Risk of P loss with snowmelt runoff relates more strongly to background Olsen P than to the level after struvite amendment.Further research is needed to determine predictors of change in solubility of added struvite P over time.At the same level of Olsen P, risk of P loss with snowmelt is lower than predicted if soils contain struvite.

Drawdown of surface soil P with repeated harvest of perennial hay reduces potential for snowmelt runoff losses.

Struvite added to soils significantly increased 0–5 cm Olsen P (147 ± 85%).

Risk of P loss with snowmelt runoff relates more strongly to background Olsen P than to the level after struvite amendment.

Further research is needed to determine predictors of change in solubility of added struvite P over time.

At the same level of Olsen P, risk of P loss with snowmelt is lower than predicted if soils contain struvite.

Too much phosphorus (P) in lakes and rivers causes algal blooms. Too little P in soils can limit the ability to grow crops. Struvite is a type of P that can be recovered as fertilizer from wastewater. This research evaluates two approaches to managing P losses with snowmelt: (1) growing perennial hay crops to reduce soil P, and (2) using struvite to maintain soil fertility in a low P input cropping system. P in runoff decreased over time with harvests of perennial hay crops. P in runoff after struvite application was lower than expected when compared to fields with similar fertility without struvite, but P in excess of crop needs remained in the soil after harvest. Both strategies offer benefits for water quality with struvite providing the added benefit of recovering P from wastewater. Careful balancing of P inputs with crop needs is required to maintain productivity but prevent accumulation of excess soil P.