Journal Paper Digests

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Journal Paper Digests 2021 #34

  • Soil carbon sequestration potential and the identification of hotspots in the eastern Corn Belt of the United States
  • Sensitivity and variability of soil health indicators in a California cropping system
  • Relating indicators to soil health functions in conventional and organic Mediterranean cropping systems

Soil carbon sequestration potential and the identification of hotspots in the eastern Corn Belt of the United States

Soil C sequestration is a significant CO2 mitigation strategy, but precise assessments of sequestration require spatially explicit modeling of potential changes in soil organic C (SOC) in response to soil, climate, land condition, and management interactions. We assessed the SOC sequestration potential of the eastern Corn Belt (ECB) in the United States (Indiana, Kentucky, Maryland, Michigan, Ohio, Pennsylvania, and West Virginia) in response to the adoption of conservation farming practices and land use change (pasture and forestation) using the SOCRATES model. Input data was provided through an intersection of the State Soil Geographic database, National Land Cover Database, and a PRISM (https://prism.oregonstate.edu) climate surface. At the end of the 20th century, the 15.3 Mha of cropped soils in the ECB contained 632 Tg C, an estimated reduction of 52% since the introduction of agriculture in the mid 1800s. Complete adoption of no-tillage practices on prime cropland would potentially recover 147 Tg SOC over 20 yr, whereas a continuation of conventional tillage would produce a loss of 35 Tg SOC over that period. Sequestration hotspots (>500 Gg increase in SOC) under no-tillage cover 2.3 Mha providing 28 Tg C over 20 yr. The conversion of marginal (nonprime) agricultural lands to forests would yield an additional 13 Tg C in SOC and 381 Tg C in aboveground biomass. The rehabilitation of minelands to forest would yield an additional 4 Tg C in SOC and 42 Tg C in biomass. Opportunities to sequester C in the ECB via tillage and reforestation are substantial and should be incorporated into regional and national climate change mitigation strategies.

Sensitivity and variability of soil health indicators in a California cropping system

An indicator that is used to monitor whether a management practice is improving soil health must be sensitive to management changes. However, it should not be overly influenced by variations in sampling time or location, previous crop, or annual differences in weather or operations timing. In this study, we assessed the sensitivity and variability of several soil health indicators in long-term plots under typical farming practices in a Mediterranean climate. These plots have been conventionally or organically farmed in a corn (Zea mays L.)-processing tomato (Solanum lycopersicum L.) rotation for 25 yr. We sampled in both crop phases prior to planting and midseason for two consecutive years, analyzing subsamples taken from three adjacent locations per plot. Management was the most significant factor differentiating most indicators, particularly indicators of biological processes and C accumulation. Whereas management differences were consistent across sampling times, average indicator values for a management system often varied significantly between dates and years. Crop phases, conversely, were usually similar. Accounting for soil texture increased management sensitivity for aggregate stability and most C accumulation indicators. Sensitive indicators such as mineral N, particulate organic matter C, and mineralizable C had greater subsample variability than indicators measuring large, stable pools, such as total C. Our results show that indicators relating to organic C and biological processes most strongly differentiated the two systems, and underline the importance of using consistent sampling dates. They also suggest that an indicator dataset including both stable and sensitive indicators may be the most reliable to interpret.

Relating indicators to soil health functions in conventional and organic Mediterranean cropping systems

Accurate assessment of the benefits of soil health building practices to soil function and crop performance requires region-specific data and locally relevant indicators. In this study, we used a long-term experiment to measure the effect of 25 yr of differing management on soil health and crop performance indicators in organic and conventionally farmed annual crops in a Mediterranean climate. We measured the strength and consistency of the relationships between several indicators and three functions of a healthy agroecosystem-C storage, net N mineralization, and yields-over two growing seasons and two crop types. Lastly, we used path analysis to test the hypothesis that healthier soils lead to healthier plants and higher yields. Organic plots had greater C stocks and net N mineralization compared with the conventional plots, but lower yields. The path analysis suggested that yields were limited by factors other than N deficiency. The relationships between soil health indicators and soil function were unaffected by crop type but were moderated by yearly changes in weather and operations timing. The indicators most strongly and consistently related to C stocks were permanganate oxidizable C and microbial biomass C, and to N mineralization were CO2 mineralized from rewet soil and fluorescein diacetate hydrolysis. Our results highlight that (a) different indicators are appropriate for assessing different aspects of soil health, (b) using several years of data in developing robust ranges for indicator interpretation is important, and (c) links between soil and crop health must be assessed mechanistically within a given system.

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