Inclusion of soil organic carbon changes can reverse results of historical energy flux quantifications of Pampean agriculture

Abstract

The questions that guided this work were: 1) How do energy inputs, outputs, and energy indices evolve during the last four decades along the rainfall gradient of the Pampas, 2) How does present agrochemical and fertilizer use in Argentina resemble, or differ, from other main grain producing regions with large no-till surfaces?, and 3) How do energy fluxes vary when soil organic carbon (SOC) changes during the last four decades are included? Energy balances (outputs – inputs), energy efficiencies (outputs/inputs) and energy intensities (inputs/yield) were calculated. Inputs comprised agrochemicals and fertilizers, machinery used for soil tillage and fuel use and gathered from different information sources. Outputs included yield of main crops from national statistics. Calculations were performed for four areas along a rainfall gradient during the 1970–2015 period. Energy coefficients were collected from literature. Soil organic carbon changes of the upper soil profile meter were available from a previous publication. Total input averages per area were low although increased 62% after four decades, from 6.6 GJ ha−1 in the past up to 10.5 GJ ha−1 at present with no marked differences between areas. Agrochemicals comprised 49% of total energy input, a very large proportion compared to other regions mainly related to the large surface under no-tillage while fertilizer rates were low. Average energy outputs increased 51% with time and all energy balances were positive. Energy efficiencies had an optimum during 1995 of 4.8 decreasing afterwards down to 3.7. Energy intensities decreased and at present 14% less input energy was needed per t DM yield produced. Two areas gained SOC and one lost large amounts. Inclusion of SOC losses in energy quantifications turned all energy indices to negative values therefore providing a real scenario of what happened with energy fluxes after four decades of agriculture which otherwise would be ignored.