Resiliency Against Agricultural Droughts and Excess Water

Agricultural production fields in Pennsylvania and many other regions in the United States have been increasingly experiencing more frequent and intense droughts and flooding. A quick analysis of total rainfall received during the summer period (June until August) reveals wetting trends across vast areas of the Northeastern U.S. and the Mid-Atlantic region counties from 1981 to 2021 (Figure 1A). At the same time, the atmosphere’s demand for water (which depends on temperatures, radiation, humidity, and wind speed) has been increasing during 1981-2021 across the region (Figure 1B). Water availability for crop growth, development, and yield production will be dictated by the balance between water supply from rainfall (under rainfed production) and water demand from the atmosphere. Besides, the within-season distribution of rainfall and its comparison to water demand is critically important in addition to the seasonal total water balance. Rainfall events have been and will continue to be higher in intensity but separated by longer and more intense dry spells. The 2022 drought in the Northeast U.S. is a recent example. Thus, agricultural ecosystems will experience dual challenges from significant excess water and low water availability conditions, resulting in further stress on agroecosystems and other natural resources.

The negative impacts of such non-ideal water availability conditions on crop yields are evident in Pennsylvania. A quick analysis of USDA survey data of corn yields in all 67 Pennsylvania counties during 1981-2021 shows that corn-growing counties and years have encountered both limited and excess water conditions during the last four decades (Figure 2). Both scenarios have resulted in the loss of yield relative to that at optimal water availability conditions (Figure 2). These local data suggest that crop productivity in Pennsylvania has been significantly vulnerable to year-to-year variation in crop water availability, leading to limitations in food production and economic loss.

Soils have a major role to play in buffering the impact of climatic water availability on crop yields. When managing to avoid crop water stress, certain soil properties such as field capacity, permanent wilting point, and saturation point are important soil health indicators to track:

1. Field capacity is the amount of water that a soil can hold after excess water has drained away and the soil has had time to settle. It is the maximum amount of water that the soil can hold before it begins to drain away. If the soil is holding more water than the field capacity, there may be excess water that can lead to waterlogging and root rot.
2. Permanent wilting point is the point at which a plant can no longer extract water from the soil. At this point, the plant will wilt and will no longer be able to maintain its normal functions. If the soil is holding less water than the permanent wilting point, the plant may not have enough water to survive.
3. The saturation point of soil is the point at which the soil is completely saturated with water and can hold no more. At this point, the pores in the soil are completely filled with water, and there is no air space left for the roots to take in oxygen. Without oxygen, the roots will be unable to perform their functions properly, and the plant will be stressed or may even die. In addition, saturated soil can lead to other problems, such as waterlogging, erosion, and the leaching of nutrients.

Variation in these three soil characteristics dictates the vulnerability of crops to water limitations and water excess conditions. The difference between field capacity and the permanent wilting point is called available water capacity, which represents the ability of soils to hold water that is available for crop uptake. Considerable variation exists in available water capacity across the state (Figure 3). While much of this variation exists because of variations in the texture of soils (proportion of sand, silt, and clay), soil structure also has an important role too.

A major pathway that offers the ability to improve these soil characteristics is by improving soil organic carbon (SOC) stocks. SOC refers to the carbon component of soil organic matter from plant residues and living microbial biomass. It is generally assumed that soil organic matter contains 58 percent SOC. SOC stocks can be sequestered by adopting conservation tillage practices, planting cover crops, implementing precision fertilization techniques, and numerous other ways, as described in the article Carbon Sequestration and Credits for Pennsylvania by Miller, White, and Larson. While direct intended potential benefits of engaging in these practices would be greenhouse gas emissions reduction and direct compensation by participating in carbon markets, sequestering carbon in agricultural soils can also add to resiliency against non-ideal water availability conditions.

So, what are the mechanisms that are responsible for improved resiliency upon improving SOC stocks? To understand this, we will have to turn to research data compiled across diverse climates, soils, and management practices across the U.S. and globally. In general, as SOC increases, water content at field capacity increases at a higher rate than that at the permanent wilting point, ultimately increasing AWC (Hudson, 1994). This is illustrated in Figure 4. The National Cooperative Soil Survey Soil Characterization database, which contains data for more than 20,000 pedons of U.S. soils, was analyzed by Libohova et al. (2018) to understand the relationship between SOC and AWC. They found that a 1% increase in SOC contributes to an average increase in AWC by 1.72%, and the increase was dependent on soil texture. Another team of researchers (Minasny and McBratney, 2017) also reported similar estimates (1.1% AWC increase per 1% increase in SOC) upon analyzing 60 published studies and more than 50,000 measurements globally. Water content at saturation point increased the most at 2.95% per 1% increase in SOC. Sandy soils benefit the most from SOC increase, followed by loams and clays. A more recent effort across North American soils showed a more pronounced effect than previous findings at a 3% increase in AWC per 1% increase in SOC on average across all soil textures (Bagnall et al., 2022).

These data support the indirect benefits of adopting climate-smart agricultural practices on improving the climate resiliency of agricultural production. Improving SOC can help in reducing crop impacts from droughts and flooding by beneficial tweaking of critical soil properties so that soils can hold a greater amount of water before flooding or limited water stress sets in. In addition, recent research has also shown that higher available water capacity soils have helped in reducing crop yield losses due to dry atmosphere conditions (Kukal et al., 2023). Realistically, increasing soil organic carbon is a slow process and will most likely require a combination of conservation and nutrient management practices over multiple years, as shown by Dr. Sjoerd Willem Duiker’s article Can I Increase Soil Organic Matter by 1% This Year?

References

• Hudson, B. D. (1994). Soil organic matter and available water capacity. Journal of soil and water conservation, 49(2), 189-194.
• Minasny, B., & McBratney, A. B. (2018). Limited effect of organic matter on soil available water capacity. European journal of soil science, 69(1), 39-47.
• Bagnall, D. K., Morgan, C. L., Cope, M., Bean, G. M., Cappellazzi, S., Greub, K., ... & Honeycutt, C. W. (2022). Carbon‐sensitive pedotransfer functions for plant available water. Soil Science Society of America Journal, 86(3), 612-629.
• Libohova, Z., Seybold, C., Wysocki, D., Wills, S., Schoeneberger, P., Williams, C., ... & Owens, P. R. (2018). Reevaluating the effects of soil organic matter and other properties on available water-holding capacity using the National Cooperative Soil Survey Characterization Database. Journal of Soil and Water Conservation, 73(4), 411-421.
• Kukal, M. S., Irmak, S., Dobos, R., & Gupta, S. (2023). Atmospheric dryness impacts on crop yields are buffered in soils with higher available water capacity. Geoderma, 429, 116270.