Radiative forcings of converting from conventional to alternative agricultural systems

Agriculture, which occupies ~35% of the Earth's ice-free land surface, alters biogeochemical cycling and the surface energy balance, which in turn can modulate regional and global climates. Corn and soybeans are the most important field crops by value in the United States, with the majority of production occurring in a ~60 million ha region in the Midwest known as the Corn Belt. There are concerns that conventional farming practices may have an adverse effect on climate because intensive tillage practices can exacerbate soil organic carbon (SOC) losses, high rates of synthetic nitrogen (N) fertilizer application increase nitrous oxide (N₂O) emissions, and the bare soil conditions that persist for 6–7 months each year alters carbon (C) cycling and the surface energy budget. Alternative systems that incorporate reduced tillage intensity, cover crops or intercropping have been proposed as possible ways to mitigate the climate impacts of conventional management. The challenge is that past field-scale research efforts have not taken an integrated approach that simultaneously considers the biogeophysical and biogeochemical radiative forcings (RF). A better understanding of the net RFs of alternative management systems is therefore needed to determine their net climate impacts, and to support the development of science based management recommendations.

Here, we use >30 site-years of data with the overall goal of determining the net direct RFs associated with changing from conventional to alternative corn-soybean systems. Specific objectives include (i) quantifying the seasonal and annual differences in surface albedos and greenhouse gas (GHG) exchange, and their associated biogeophysical and biogeochemical RFs, (ii) quantifying the net RFs of each management practice, and (iii) re-evaluating the mitigation potential of alternative management systems. We are examining several systems to explore the range of mitigation possibilities including: (i) conventional tillage and N fertility, (ii) annual spring cover crops with strip tillage, and (iii) cover/intercropping perennial kura clover (Trifolium ambiguum M. Bieb.) with strip tillage. We hypothesize that increasing the length of vegetated surface conditions through cover crops will induce a negative net RF (i.e., cooling), largely driven by the step change in albedo caused by the presence of plants and their residues during the non-corn growing season.

In our approach, we consider conventional management the ‘reference' and calculate the RF induced by converting to alternative systems, where positive and negative RFs reflect warming and cooling, respectively. We calculate the traditional global RFs, as well as the local albedo RF because this is a better indicator of regional-scale effects that can overwhelm the global signal. In this presentation, we will describe our experimental approach, and focus the presentation of results on the radiation data and analyses of the albedo RFs. A preliminary analysis of GHG RFs will also be presented.