Soils are the largest sink of carbon (C) on Earth and store ~1500 petagrams (1 Pg = 1015
g) of C up to 1 m depth, which is two times higher than the atmosphere (750–950 Pg C) and three times higher than total vegetation (600 Pg C) [1
]. A part of the stored C in soils is lost to the atmosphere as carbon dioxide (CO2
) and methane, which contributes to greenhouse gas emissions. Agricultural activities such as enteric fermentation in animals, manure management, urea fertilization, and liming accounted for 619 million metric tons of CO2
equivalents or ~10% of total greenhouse gas emissions in the United States in 2018 [4
], whereas enteric fermentation and manure management contributed 28% and 10% of total methane emissions from anthropogenic activities, respectively. Other agricultural soil management activities, such as fertilizer application, deposition of livestock manure, and growing N-fixing plants contribute more than 75% of nitrous oxide (N2
O) emissions. On the other hand, there are positive aspects of agriculture. For example, it was estimated that global croplands could sequester 0.90 to 1.85 Pg C/yr; this corresponds to 26% to 53% of the soil C sequestration target of the “4 per 1000” initiative established for climate mitigation [5
]. Thus, enhancing soil organic C (SOC) sequestration using agricultural management practices can potentially mitigate a part of the greenhouse gas emissions.
Multiple factors, such as land use, soil type, and climate, influence SOC [6
]. Conversion of native land to agricultural land often results in a decline in SOC due to an increase in erosion, decomposition, and leaching [8
]. Sanderman et al. [10
] used data collected over a 50-year period and estimated that globally 133 Pg of C was lost from the upper 2 m of soil due to agricultural land use. It is widely known that the amount of SOC is greater in surface than sub-surface layers and in fine-textured than medium- and coarse-textured soils [11
]. Further, the amount of SOC generally increases with an increase in annual precipitation due to the availability of more water for plant growth and a decrease in temperature due to a reduction in the decomposition of organic matter [6
Soil quality is linked to the physical, chemical, and biological properties of soil, and is affected by management practices. Recent reviews and meta-analyses have provided insights on how agricultural management practices, such as tillage, crop rotations, cover crops within rotations, and fertilization management, alter the amount of SOC [12
]. For example, a meta-analysis by Poeplau and Don [13
] concluded that the use of cover crops led to a significant increase in SOC. Accumulation of SOC is associated with physical protection and chemical interactions with clay minerals [15
]. Many field studies have shown that no-tillage generally increases soil organic matter (which contains ~58% of SOC) and aggregate stability in the upper soil layer compared with conventional tillage [16
]. Luo et al. [18
] synthesized the data from 69 global studies and reported that no-tillage enhanced SOC stocks in the top 10 cm of soil, but decreased SOC in the 20 to 40 cm layer, and did not increase SOC below 40 cm. Other meta-analysis indicated that SOC stocks can vary at soil depths between no-tillage and tillage systems [19
]. Angers and Eriksen-Hamel [19
] pointed out that greater SOC content may occur at the bottom of the plow layer under full-inversion tillage. In terms of fertilization, manure application is the most effective way to increase SOC sequestration in agricultural fields [21
]. Studies have suggested that it could take several years (more than five years) to change the overall soil functional capacity and determine a significant change in SOC after the alteration of management practices [22
The carbon to nitrogen (C/N) ratio influences microbial activity in soils and has been used to estimate the quality of soil organic matter and the subsequent decomposition [23
]. In general, the C/N ratio of soil has a negative relationship with the decomposition of soil organic matter [24
]. Stable C isotopic composition (δ13
C) in SOC has been widely used to investigate the sources of organic C and relative contribution of C3 and C4 plants to SOC [25
]. Thus, the use of isotopic techniques may allow us to determine SOC dynamics in agricultural soils under different management practices.
Variability in the methods used to assess the changes in SOC stocks present a challenge to accurately determine the magnitude and direction of SOC stock change in agricultural soils. Understanding the current amount and spatial distribution of SOC can help to quantify and track C, which can help to sequester more C in soils to mitigate climate change concerns. Knowledge of a precise amount of SOC can then be used by policymakers to incentivize the adoption of practices that will maximize SOC stocks. At the state level in Maryland, United States, the legislation passed a bill in 2017 to promote soil health programs and practices [26
]. This policy defined a need to develop local capacity to measure SOC and other soil health metrics for the successful implementation of soil health programs. Ellert et al. [27
] and Conant et al. [28
] suggested a “benchmark site” approach that can be optimized to quantify SOC stocks change over time.
Our objective in this study was to use a grid sampling approach to assess the magnitude of SOC variability and determine the current (baseline) SOC stocks in three typical agricultural fields in Maryland. The results from this study may provide information on ways to quantify and track SOC and how different management practices can be implemented and optimized to enhance C sequestration and increase SOC stocks in soils.