Carbon budget of an intensively grazed temperate grassland with large quantities of imported supplemental feed

Abstract

Abstract Management of agricultural grasslands can alter soil carbon (C) stocks, and quantifying the effects of different management regimes is important to inform global greenhouse gas (GHG) mitigation strategies. Here, we report the net ecosystem carbon balance (NECB) over three years for an intensively managed rotationally grazed New Zealand dairy farm importing 11.6\u2009t DM ha−1\u202fy−1 (526\u202fg\u202fC\u202fm−2\u202fy−1) as supplemental feed. We investigated whether importing large quantities of supplemental feed to increase milk production led to a gain in soil C. Eddy covariance measurements were used to quantify net ecosystem productivity (NEP), with all other imports and exports of C calculated from farm records and measurements. Additionally, we compared the NECB calculated with a system boundary equivalent to the farm boundary (NECBFarm), to that calculated with the system boundary around the paddocks in which the EC footprint extended (NECBFootprint). Both the three-year average NECBFarm and NECBFootprint were similar, but could not be distinguished from zero (71\u2009±\u200977\u2009g C m−2 y−1 and 56\u2009±\u200977\u2009g C m−2 y−1, respectively; uncertainties are standard deviations). The farm boundary approach made better use of available C import and export data (excluding NEP) data, however, error propagation resulted in almost identical uncertainties of the C balance for both methods. Supplementary feed was the major C input of NECBFarm but was only a minor component of NECBFootprint due to the use of a dedicated feed pad outside of the NECBFootprint boundary. In contrast, transfer of C from excreta (including that resulting from the supplementary feed consumed on the feed pad) was the major C input of NECBFootprint. Consumption of the supplementary feed by grazing animals resulted in most of the imported C exiting the system boundary via pathways that included the export of product (milk), respiration of CO2 and methane production. Only the imported supplementary feed C excreted by the grazing animals along with the C in any feed that was wasted was available for sequestration. When coupled with manure C retention coefficients only a small gain in soil C could be expected. Our result demonstrated that importation of a large quantity of supplemental feed C did not lead to a large gain in soil C. Production of supplemental feed off site likely contributed to considerable GHG production and full life-cycle assessment is needed to determine the overall consequence of increasing milk production with supplemental feed.