Dryland cropping system intensification in the West-Central Great Plains: Impacts and barriers to adoption
As demonstrated in long-term experiments, dryland cropping system intensification (reducing the frequency of fallow in crop rotations) has the potential to increase soil carbon (C), aggregation, and farmer profitability. However, these impacts have not been observed on working farms in the West-Central Great Plains, nor have they been examined across a range of variability in climatic, soil, and management factors that influence soil properties and crop yields. Additionally, cropping system effects on microbial communities, and arbuscular mycorrhizal fungi (AMF) in particular, are poorly understood. We have been studying 96 dryland no-till fields from southeastern Colorado to northwestern Nebraska representing every level of cropping system intensity from wheat-fallow to continuous, 4-year rotations. The goal of this study is to quantify the effects of intensification on crop yields and soil properties related to carbon sequestration and resilience to climate change, and to determine how robust these affects are to variability in climatic, soil, and management factors.
Despite the suggested benefits of dryland cropping system intensification, wheat-fallow remains a ubiquitous practice in the West-Central Great Plains. Through in depth interviews with 24 producers representing every level of cropping intensity, we hope to uncover the barriers to intensifying, and the strategies and motivations that have driven successful transitions away from fallow.
Objective 1: Identify the social, ecological, political, and economic barriers to cropping intensification and the resources needed to eliminate or reduce the frequency of fallow in crop rotations.
Sub-objective 1: Examine the strategies farmers have employed to successfully intensify their cropping systems, and the obstacles faced by farmers with higher frequencies of fallow.
Sub-objective 2: Identify the factors that influence farmer decision-making at multiple scales.
Sub-objective 3: Identify the most effective educational, social, or political resources that could be mobilized to enable producers to intensify crop rotations.
Objective 2: Quantify cropping intensity effects on wheat and annualized grain yields, soil structure, and AMF across management and climatic gradients.
Sub-objective 1: Compare winter wheat yields and annualized grain yields across farms with varying cropping intensity.
Sub-objective 2: Quantify cropping intensify effects on aggregate size and stability, intra-aggregate organic carbon, bulk density, and soil organic carbon.
Sub-objective 3: Assess the effect of AMF abundance on soil aggregation dynamics.
Objective 3: Quantify the relationships between cropping intensity, mycorrhizal colonization, and drought tolerance in winter wheat.
Sub-objective 1: Quantify % AMF colonization of winter wheat roots and its effect on plant phosphorus (P) uptake.
Previously, we sought to measure plant canopy temperature as an indicator of drought stress, but we assessed leaf water potential instead. After one sampling at the most drought-prone site, we concluded there was no measurable pre-dawn or midday drought stress. We have since changed this objective (Sub-objective 1 of Objective 3) to examine the effect of % AMF colonization of winter wheat roots on plant P concentration.
Since 2015, all sample collection has been completed on the 96 fields in the study. We analyzed soil organic C down to 20 cm, and almost completed organic C analysis on 10 cm depth samples. We have assessed % AMF colonization in 140 samples, about 60% of the total. We conducted 24 in-depth farmer interviews, and coded the transcripts so that we may qualitatively analyze the data.
After accounting for soil texture, slope position, # of years in no-till, and potential evapotranspiration (PET), continuous rotations stored about 3 Mg organic C ha-1 more than wheat-fallow down to 20 cm (p<0.05), and there were no significant differences in organic C between wheat-fallow and medium intensity (2 crops in 3 years, or 3 crops in 4 years) rotations. Additionally, we assessed % AMF colonization of wheat roots on a subsample of 55 fields that grew the Byrd wheat variety. We found that % AMF colonization increased from 8% in wheat-fallow, to 17% in medium-intensity rotations, to 24% in continuous rotations (p<0.05). Total soil fungal biomass down to 10 cm, estimated by phospholipid fatty acid analysis, displayed a similar trend in a subset of 72 fields, with no significant difference between continuous rotations and 30-year shortgrass prairie strips. After accounting for annualized fertilizer nitrogen (N) use and PET, annualized grain yield from 2010-2014 increased with cropping system intensity (p<0.01), to max out in rotations with one fallow period in five years. These data were collected from a subset of 46 fields from working farms. In the coming months, we will be adding 2015 and 2016 yield data to these results, and isolating the effect of fallow elimination on farm-wide winter wheat yields.
Impacts and Contributions/Outcomes
We presented preliminary results at the 2016 ASA, CSSA, and SSSA conference in Phoenix for which we received an award (S. Rosenzweig, M. Schipanski. How robust is the rotation effect in semi-arid systems? ASA, CSSA, SSSA Annual Meeting 11/8/16). We also presented the results in two guest lectures for classes, and at a seminar for the Soil and Crop Sciences department at Colorado State University. We completed filming for a short film based on this study, and purchased a domain and began to populate a website (drylandag.org). Preliminary results will be presented at a regional farming conference at the end of January, and we expect to release the short film in February.