Greenhouse Gas Emissions from Subtropical Agriculture Fields Decrease Over Time

Abstract

Expansion of cultivated lands and field management impacts greenhouse gas (GHG) emissions from agriculture soils. Soils naturally cycle GHGs and can be sources or sinks depending on physical and chemical properties affected by cultivation and management status. We looked at how cultivation history influences GHG emissions from subtropical soils. We measured CO₂, N₂O, and CH₄ fluxes, and soil properties from newly converted and continuously cultivated lands during the summer rainy season in calcareous soils from south Florida. Newly converted soils had more soil organic matter (OM), more moisture, higher porosity, and lower bulk density, leading to more GHG emissions compared to historically cultivated soils. Although more nutrients make newly converted lands more desirable for cultivation, conversion of new areas for agriculture was shown to release more GHGs than cultivated lands. Our data suggest that GHG emissions from agricultural soils may decrease over time with continued cultivation.

1. Introduction

Greenhouse gas emissions from agriculture are an important environmental impact to consider. Plant and livestock production currently account for 13% of global greenhouse gas (GHG) emissions (EPA, 2020). Fertilizer use, tillage, and irrigation contribute to carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) emissions (Tilman et al., 2001), yet are necessary to support continued cultivation of soils. Humans convert new land for agriculture because of issues with soil fertility and climate change (Galford et al., 2010). Such changes in land cover and land-use can make up 24 to 49% of the GHG budget associated with agricultural land management (Galford et al., 2010). While extensification, the cultivation of new land, is one way to maintain or increase production, intensification, or increasing inputs per area, can support continued cultivation of land (Pellegrini & Fernandez, Pellegrini & Fernández, 2018). Farm management practices, such as the use of cover crops and conservation tillage can improve soil fertility allowing for continued cultivation and possibly reduce GHG emissions (Kallenbach et al., 2010; Snyder et al., 2009; Tully & Ryals, 2017). Considering agriculture is a significant contributor to GHGs, there is a need to understand how land cultivation and farm management practices impact GHG emissions from agricultural soils.

2. Objective

The objective of this study was to compare soil characteristics and GHG emissions between newly converted and historically cultivated subtropical agricultural soils. GHG measurements from soils located at the University of Florida Institute of Food and Agricultural Sciences Tropical Research and Education Center in Homestead, Florida, USA are relevant for future large-scale research in agroecosystem management in south Florida and other regions with similar calcareous soil type and agricultural management. More broadly, information on GHG emissions associated with cultivation status and history of agricultural soils can help quantify future GHG emissions associated farm management.

3. Methods

Krome gravelly loam soils (Nobel et al., 1996), were sampled once in May, July and August 2018 for GHG emissions and soil moisture in four 20m x 20m plots. Two plots were historically cultivated for vegetable production (> 20 years). The other two plots were covered in turf and scattered trees until the top 0.30m of oolithic limestone rock was crushed and plowed in March 2018. Plots used in this study were neither fertilized nor irrigated but divided into 36 sub-plots and planted with multiple cover crops. Vented static flux chambers (Holland et al., 1999) were randomly installed at 4 sub-plots in each of the 4 larger plots at a depth of 2-3 cm. Headspace samples (20 mL) were taken 0, 15, 30, and 45 minutes after chambers were capped and analyzed for CO_2, N₂O, and CH₄, using a gas chromatograph with ECD and FID detectors. Fluxes were determined from the four time points using linear regressions fitted to the changes in concentration over time when the regression line had an R² > 0.65. Our minimal detectable flux for CO₂, CH₄, and N₂O was 1.98, 1.70, and 2.04 g ha^‒1 day^‒1, respectively. N₂O and CH₄ were converted to carbon dioxide equivalents (CO₂ eq.) using conversion factors 298 and 25, respectively. Soil samples (n=36) were collected from each sub-plot of the four plots at 10 cm depth and analyzed for organic matter (OM), extractable ammonium (NH₄⁺) and nitrate (NO₃^‒) in March 2018. Soil porosity and bulk density were collected from all sub-plots in one cultivated and one converted plot. Mixed effects models were used to compare GHG fluxes and soil properties between cultivation history.

4. Results

A majority of the CH₄ (37/48) and N₂O (40/48) fluxes were below the detection limit; detectable fluxes (CH₄ (10/11) and N₂O (8/8)) were from newly converted soils. The average flux for CH₄ and N₂O was 0.0013 and 0.0036 kg ha^‒1 day^‒1, respectively, while the average CO₂ flux was 12.66 kg ha^‒1 day^‒1. In CO₂ eq, CH₄ and N₂O accounted for 1.5% of the overall CO₂ emission. CO₂ eq were 73% higher in newly converted soils compared to cultivated soils (Figure 1). Soil OM and soil porosity were significantly higher in newly converted soils compared to cultivated lands (Table 1). Cultivation decreased soil moisture compared to converted soils (Table 1). Bulk density was lower in newly converted soils than cultivated soils. Soil NH₄⁺ and NO₃^‒ content were approximately 2.5 times and 8 times higher in the converted soils, respectively.

Figure 1. Mean (±SE) CO₂ Equivalent fluxes (sum of CO₂ and non-CO₂ fluxes) from converted and cultivated soils (F_1,2 = 13.82 p = 0.065). Fluxes were measured three times over 4 months during the summer rainy season.

Table 1. Summary table of results for soil physical and chemical properties (± SE). Different letters indicate mean values are significantly different (p < 0.05).

5. Discussion

Land conversion to agriculture in subtropical calcareous soils had higher GHG emission shortly after conversion than soils historically used for subtropical agriculture. Given the soil type and surface bedrock, we were unable to have an undisturbed control plot. Higher nitrogen content and soil moisture, as found in the newly converted soils, are both driving factors for GHG production in soils (Snyder et al., 2009). Lower porosity and higher bulk density in the cultivated soils might reflect the frequent tillage, compaction, and loss of organic matter, compared to the newly converted soils, which were only tilled once prior to the experiment. The recently converted soils had higher OM, NO₃^‒, and NH₄⁺ content, which is more desirable for agriculture in the short term.

6. Conclusions

Conversion of land for cultivation breaks up the structure of undisturbed soils and disrupts natural biogeochemical cycles. Soil structure and nutrient availability in converted lands may be desirable, at least in the short term despite the increase in GHG emissions. Farm management practices that facilitate nutrient storage (i.e., cover crops, conservation tillage) could allow for continued cultivation of lands and help reduce GHG emissions from agricultural soils (Galford et al., 2010; Tully & Ryals, 2017). Our results suggest that GHG emissions may decrease overtime as subtropical land is continuously cultivated and soil OM and porosity decrease. Additional studies that assess the indirect as well as direct sources of GHG emissions from subtropical agriculture are needed to accurately identify farm management practices that can decrease the GHG costs of agriculture (Gelfand & Robertson, 2015).