The growing cognizance of scientists and producers on soil health and its implication on agricultural sustainability has propelled research works on creating management strategies for improving soil health [1
] in various agroecosystems. Given the vast ecosystem services grazing lands provide [4
], more effort is needed toward the development of soil health-focused grazing management strategies. The United States has about 308 million ha of grazing lands, which is approximately 31% of the total land area [7
]. This area provides numerous ecosystem services and is an important contributor to the national GDP (gross domestic product). The Southeast region has approximately 11.6 million ha grazing lands [9
] and is home to 20% of the national beef-cattle herd [10
]. Moreover, between 2005 and 2015, almost 4000 ha of row-crop land was converted to intensive grazing farms in Georgia [11
]. Cow-calf production in pastures is common in this region due to climatic suitability and forage availability [12
], however, most of the beef-pastures are in fragmented marginal land that is vulnerable to erosion and usually not suitable for row-crop production [10
]. Thus, there is a need for more studies to better understand these livestock systems in an effort to create grazing management systems that are more sustainable and suitable for these ecoregions. In the Southeastern USA, we define conventional grazing systems as pastures continuously grazed with little control over grazing time in specific locations within pastures. This results in cattle-preference to certain areas [13
], leading to uneven nutrient distribution [15
] and inefficient land utilization. High cattle activity near pasture equipages (water, shade, hay, mineral blocks, etc.) in the conventional system results in nutrient hotspots, soil compaction [17
], and erosion resulting in poor fertility and vegetative cover in these steep, marginal areas of the pastures which exacerbates the ecosystem’s vulnerability to runoff losses.
In a recent study, it was suggested that the US cattle industry was significantly affected by drought conditions in 2017 [19
]. Additionally, high-intensity rains caused by hurricanes can increase the amount of runoff resulting in further loss of soil and nutrients. In changing global climatic scenario, a greater number of extreme weather events such as droughts and hurricanes are predicted to occur more frequently [20
], which might have overwhelming impacts on these already marginal lands.
Soil health assessment is foundational in determining the sustainability of grazing lands, thus an improvement, or at least maintenance, of the current state of soil health is important for improving the overall sustainability of pastoral systems [21
]. The soil health indicators we measured in this study have been reported as reliable as well as sensitive to management changes in agroecosystems. The active carbon fraction measured as POXC (permanganate oxidizable carbon) [24
] and soil respiration are recommended measures of soil health [25
]. A study in Georgia cattle pastures [27
] reported a range of 201–1468 mg kg−1
POXC at 0–5 cm soil depth and it has been reported [27
] that a larger POXC pool is an indicator of a healthy and more sustainable system. Mineralization of the mineralizable nitrogen pool to the plant available form and effective plant uptake also indicates a grazing system that is more sustainable [22
]. We expect an increase in plant-available nitrogen, lower runoff-nitrogen, and improved forage productivity. Pilon et al. [30
] reported that average nitrate losses from continuously grazed pastures and rotationally grazed pastures with fenced riparian buffer ranged from 0.3–1.8 kg NO3—
and 0.03–5.53 NO3—
N kg ha−1
, respectively. The Pilon et al. [30
] study was conducted in broiler litter fertilized pastures and authors were unsure of the underlying cause of higher nitrate in rotational pastures. They speculated that higher forage biomass in rotational pastures trampled during the wet period contributed to increased nitrogen losses. We expect to reduce runoff-nitrate by using better management practices.
Our study hypothesized that a collection of better grazing management practices would improve soil health indicators and reduce runoff-nitrogen as compared to the existing conventional grazing systems or rotational grazing only in Southeastern USA.
The results are in agreement with our hypotheses that strategic grazing improves or at least maintains soil health in pastures and reduces nitrate losses via runoff. Usually, conventional beef pastures are continuously grazed marginalized lands and pasture equipages such as water, shade, mineral, and hay are stationary/permanent [42
]. The grazing systems we proposed were a collection of several better grazing practices. A historical study [44
] did not find any improvement from rotational grazing in cattle productivity. However, more recent studies assessing rotational grazing systems or better grazing management systems have found positive benefits in terms of forage and animal productivity and soil health parameters [43
]. Improved water quality and soil health from cattle exclusion of vulnerable areas in pastures have also been reported in previous studies [6
]. Lure management of cattle using pasture equipages have been used to distribute the cattle activity in pastures [14
] and has the potential to distribute animal manure in the pasture while also protecting riparian areas from nutrient build-up.
The weather during this research likely had a significant impact on the results of this study. The year extremes of 2016 in terms of temperature and precipitation, as shown in Figure 2
, resulted in an eight-month drought with unusually high air and soil temperatures and an extreme negative annual soil water balance. This was followed by a relatively wet year in 2017, which included hurricane “Irma.”
The active fraction of soil carbon, measured as POXC, was a highly sensitive indicator of soil health in these grazing systems. There was a significant reduction in POXC in the year 2017 in both treatments, at all depths; however, STR pastures had a significantly greater POXC as compared to CHD pastures at all depths. In the year 2018, POXC in STR pastures was higher than the baseline (44.35, 9.29 and 18.36 mg kg−1
at 0–5, 5–10, and 10–20 cm, respectively) though not statistically significant. In the STR pastures, we speculate that there was a downward movement of POXC beyond 20 cm due to improved forage and root growth, darker soil colors, and the presence of mycelial networks that were not there in 2015 (Figure 9
). However, this information was not quantified because the sampling depth was only 20 cm. Another part of this study including more soil samples has shown a significant increase in POXC in the STR pastures at all three depths under consideration. As noted in the results, POXC in CHD pastures also recovered from extreme events but not to the extent that STR pastures recovered. This result highlights the resilience and ability of both systems to improve carbon sequestration by increasing the active carbon pool in soils at deeper depths thereby improving the overall volume of soil (increased depth of activity) in which the rhizosphere of the grassland is active. The positive trend of POXC, at all soil depths, in the STR grazing system is promising and stirs the need for longer-term research with a periodic sampling below 20 cm.
Soils are dynamic living systems, and soil respiration is a crucial biological indicator of soil health [25
]. Although difficult to measure, in situ soil respiration, measured using static chambers, was a sensitive indicator of management changes in pastures. The average soil respiration across years and grazing systems in our research was 1092 mg CO2
, which is lower than the 2628 mg CO2
reported by Chiavegato et al. [51
] during July–August in grass-legume mixed pastures in northwest Michigan. However, the comparison of soil respiration with other studies is complicated because of the differences in climate, forage type, soil type, temperature, and moisture regimes. During the baseline and the year 2017, the CHD and STR systems had similar soil respiration. However, in 2018, the STR system had significantly more (235 mg CO2
) soil respiration as compared to the CHD system. Both systems underwent a stress period during the drought of 2016 and a wet period of 2017 resulting in increased soil respiration and a reduction in POXC. Previous studies [39
] have reported increased soil respiration in wet periods that followed drought. The CHD system experienced a significant increase in soil respiration in the year 2017 and a significant reduction in the year 2018. Whereas, the STR system also experienced an increase in soil respiration in the year 2017; however, it reverted to the baseline level. We have a reason to believe that the STR system performed well because it had low variation in soil respiration across years as compared to the CHD system.
The PMN and IN fraction of soil nitrogen are crucial for any agroecosystem because nitrogen remains one of the most critical and expensive agricultural inputs. These results (Figure 6
) illustrate the ability of STR systems (exclusion and overseeding of vulnerable areas) to mineralize the PMN pool in areas that initially were more compacted from heavy animal traffic [53
] and N was not as plant available to areas that are more productive with more readily available N for plant uptake. The increase in plant-available N might be attributed to improved root growth and ground cover in the overseeded exclusions. It has been reported [54
] that most of the nitrogen in the excluded riparian areas can be lost via denitrification, thus overseeding of excluded areas critical for utilizing the nitrogen. The available nitrogen can be readily utilized by the overseeded forage helping to prolong annual grazing duration [55
]. In another part of this study, Subedi et al. [53
] noted an approximately 4% reduction in loss-on-ignition carbon during a three-year period. The mineralization of soil organic carbon releases nitrogen in the process which might also be attributed to the overall increase in inorganic nitrogen in both systems.
Nutrients in the runoff, especially nitrogen and phosphorus, are the leading cause of eutrophication and groundwater contamination [56
]. Surface deposition of feces and urine and associated nitrogen in low-lying portions of pastures that have high cattle activity are prone to runoff losses [15
]. However even with the greater concentration of inorganic N in the 0–5 cm soil layer of the STR system, runoff-nitrate was significantly reduced (from 0.17 to 0.08 kg NO3−
) which can be attributed to controlled cattle activity in low-lying vulnerable areas, improved ground cover in the AOIs, deeper root growth, and plant utilization of the available nitrogen. The CHD pastures had a small reduction in runoff nitrate after treatment, but it was not statistically significant. Before treatment, per unit increase in time spent by cattle, within 50-m the runoff collectors, would result in 0.27 kg ha−1
increase in runoff nitrate. After the treatment, in the STR system, there was no effect of cattle locus on runoff nitrate which shows the efficacy of controlled utilization (overseeded and limited grazing) of low-lying areas vulnerable to erosion in reducing runoff-nitrate losses. Previous studies [57
] have reported the effectiveness of riparian buffer in agricultural watersheds to protect and utilize nitrate. It should be noted that exclusions were not placed adjacent to streams, but they were in concentrated flowpaths adjacent to riparian areas which had little vegetation within the concentrated flowpaths. The exclusions in this study not only protected the nitrogen [60
] from leaving the field but also provided extra forage [55
] where little to no vegetation was present during baseline. During the baseline, the positive relationship between soil-nitrate and runoff-nitrate indicated that regions with greater soil-nitrate tend to lose more nitrate in runoff. After the treatment, in STR pastures, that relationship was not evident, again illustrating the effectiveness of flash grazing of overseeded exclusions in vulnerable areas for improving surface water quality.
Strategic grazing has several evident advantages in terms of soil health and surface water quality over the existing conventional grazing system and its slightly improved version, the CHD system. Moreover, the ability of the STR grazing management system to recuperate from extreme weather events such as droughts and hurricanes, in terms of soil health, establishes the potential of this system to improve the overall sustainability of managed pastoral systems in the changing climatic scenario.