Materials and Methods

Sampling was conducted in eight counties throughout the coastal plain of South Georgia in 2020 and 2021 (Fig. 1). This is a major region of commercial pecan production. The conditions observed throughout the sampled area exemplify agricultural soils of the southeastern U.S. coastal plain (Wells, 2009). Commercial pecan orchards established from land previously in row crop production and located directly adjacent to row crop fields were selected for sampling. Adjacent row crop fields were also sampled to determine differences in soil quality with minimal variation in soil type and texture. Pecan orchards were separated into age groups of 1–4, 5–10, 11–20, and >20 years old to analyze differences in soil quality with orchard age. Orchards and corresponding row crop fields sampled across all sites were largely composed of seven different soil types. These include Tifton (fine-loamy, kaolinitic, thermic Plinthic Kandiudults), Dothan (fine-loamy, kaolinitic, thermic Plinthic Kandiudults), Fuquay (loamy, kaolinitic, thermic Arenic Plinthic Kandiudults), Greenville (fine, kaolinitic, thermic Rhodic Kandiudults), Faceville (fine, kaolinitic, thermic Typic Kandiudults), Orangeburg (fine-loamy, kaolinitic, thermic Typic Kandiudults), and Red Bay (fine-loamy, kaolinitic, thermic Rhodic Kandiudults). All of the orchards selected for sampling are managed for commercial production in accordance with University of Georgia Cooperative Extension recommendations (Wells, 2017a). Row crop fields sampled followed conventional farming practices with both conventional and conservation tillage methods. These fields were either in cotton, peanut, or corn production each year.

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Soil sampling was conducted from 1 June to 31 July in 2020 and from 1 June to 30 July in 2021. Soil samples were taken at random throughout each pecan orchard and row crop field. Samples were collected from the row middles of orchards and row crop fields at a depth of 0–15 cm each year. Four composite samples composed of four soil cores per composite were collected from each pecan orchard and each corresponding row crop field. In 2021, an additional round of soil samples was collected at a depth of 91–106 cm. Cores taken at this depth were combined to create one composite for each pecan orchard and corresponding row crop field. A total of 496 composites were collected throughout 41 commercial pecan orchards and row crop fields across the sampled region.

Soil was analyzed by Waters Agricultural Laboratory (Camilla, GA) each year. Samples were dried and sieved using a 2-mm mesh before analysis. Soil quality indicators were tested at 0–15 cm in both years and 91–106 cm in 2021. Soil pH was determined with a soil/water ratio of 1:1 and read with a hydrogen probe. Organic matter was determined by the loss of ignition method and is expressed as a percentage by weight. Total nitrogen was determined using the Kjeldahl method. CEC was determined by the ammonium acetate method (pH 7). Aggregate stability was determined using the Volumetric Aggregate Stability Test (Woods End Laboratories). Bulk density and porosity were calculated using the core method. Solvita CO₂-Burst and labile amino nitrogen (SLAN) tests were conducted according to Woods End Laboratories guidelines (Brinton, 2019a, 2019b). Active carbon measurements were determined following UIUC Soils Laboratory POXC procedure (SOP. POXC 2021).

Analysis of variance (ANOVA) was used to compare measurements of soil quality indicators from pecan orchards at different age ranges and adjacent row crop fields. All statistical analyses were performed using SigmaPlot 14 statistical software. All pairwise multiple comparison procedures were performed using Tukey’s test (P < 0.05). Linear regression was performed to examine the relationship between orchard SOM and active carbon, active carbon and Solvita CO₂-Burst, and orchard SOM and Solvita CO₂-Burst at 0–15 cm.

Results and Discussion

Significant differences in SOM were observed between sites with samples taken from 0 to 15 cm. SOM was significantly higher (P < 0.001) in orchards 11 years and older than that of row crop fields in 2020 (Table 1). Orchards from the 11–20 and >20-year-old groups contained 1.08% and 1.85% SOM, respectively, whereas the row crop fields contained only 0.73%. In 2021, soil analysis showed that organic matter was significantly higher (P < 0.05) in orchards 1–4 and >20 years old compared with row crop fields (0–15 cm) (Table 1). Orchards 1–4 and >20 years old contained 0.99% and 1.17% SOM, respectively, whereas row crop fields contained 0.71%. Samples taken at 91 cm provided a measure of the more stabilized amount of organic matter present in the soil profile throughout these sites. These values from deeper in the soil profile have greater potential significance with regard to long-term carbon storage (Tautges et al., 2019). At this depth, mean values of orchard soils ranged from 0.84% to 1.03% organic matter while row crop fields contained an average of 0.98%. There were no significant differences in the amount of SOM present throughout orchards and row crop fields at a depth of 91 cm in the soil profile (Table 2). Variations in organic matter between 2020 and 2021 at the 0–15 cm depth are likely related to microbial activity as influenced by variations in temperature and/or rainfall. The southeastern United States is more susceptible to organic matter decomposition due to the warm and wet climate (Triplett and Dick, 2008). Such climates are particularly favorable for microbial activity throughout the majority of the year, which prevents extensive accumulation of organic matter (Wells, 2009). However, compared with conventionally farmed row crop fields from the same region, pecan orchards do tend to hold a higher percentage of organic matter (Giddens, 1957; Wells, 2009). Higher levels of organic matter in pecan orchards are likely the result of the accumulation of plant biomass that is returned to the soil each year. According to Idowu et al. (2017), pecan husks alone account for 25–30% of the total mass of the pecan fruit. These husks dry out and fall to the orchard floor each growing season, bringing a considerable amount of plant material back to the soil. Decomposition of leaves and woody plant debris on the orchard floor (Wells, 2009) along with orchard soils being left uncultivated (Giddens, 1957) also contribute to the increased levels of organic matter observed in pecan orchards.

Table 1.Soil quality indicator results from soil samples taken in row crop fields and pecan orchards of 1–4 years of age, 5–10 years of age, 11–20 years of age, and over 20 years of age in 2020 and 2021 at 0–15 cm in depth.

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Table 2.Soil quality indicator results from soil samples taken in row crop fields and pecan orchards of 1–4 years of age, 5–10 years of age, 11–20 years of age, and over 20 years of age at 0–15 cm and 91–106 cm for each site in 2021.

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In 2020, active carbon was significantly higher (P < 0.001) in orchards 5 years and older compared with row crop fields (Table 1). Active soil carbon from orchards 5–10, 11–20, and >20 years old measured 405.28 mg·kg⁻¹, 485.19 mg·kg⁻¹, and 715.05 mg·kg⁻¹, respectively, while row crop fields measured 333.47 mg·kg⁻¹. In 2021, active carbon at 0–15 cm was significantly higher (P < 0.001) in orchards 11 years and older than that of row crop fields (Table 1). Orchards from the 11–20 and >20-year-old group measured 322.95 mg·kg⁻¹ and 476.85 mg·kg⁻¹ respectively, while row crops contained 246.13 mg·kg⁻¹. Active carbon levels were significantly lower (P < 0.001) in samples taken at 91 cm. We observed no significant differences in the amount of active carbon between orchards and row crop fields at this depth (Table 2). The active portion of soil carbon has been found to strongly correlate with improved microbial activity and other soil carbon functions (Culman et al., 2012; Weil et al., 2003). Orchards from the >20-year-old group exhibited 114% and 94% more active carbon in 2020 and 2021, respectively, than row crop fields at 0–15 cm. These data indicate that microbially available energy sources increase in pecan orchards with orchard age. Active carbon generally has a strong relationship with total SOM (Breker, 2021a), but it responds much sooner to differences in crop and soil management (Hurisso et al., 2016). We found active carbon to be positively correlated (r = 0.81) with SOM (Fig. 2). The coefficient of determination value for this relationship indicates that the SOM percentage explains 66% of the variation observed in active carbon in this study (Fig. 2). Observed SOM data does show significant increases in orchard soils, but the overall amount is still relatively low (<2%). The relatively low SOM levels in the Georgia Coastal Plain compared with soils in other regions are a result of the highly weathered soils, the warm, humid climate, and the resulting rapid breakdown of SOM by soil microbes. Breker (2021a) suggests that active carbon measurements help explain why soils with similar levels of organic matter have the ability to exhibit much different levels of biological activity.

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The Solvita CO₂-Burst test was used to evaluate soil respiration as a result of microbial activity. Solvita CO₂-Burst measurements were significantly higher (P < 0.001) in orchards 1–4 years old and older compared with row crop fields in 2020 (Table 1). Orchard soils exhibited an increase in CO₂-Burst values as they increased with age (Table 1). Mean CO₂-Burst values ranged from 39.12 mg·kg⁻¹ (1- to 4-year-old orchards) to 54.49 mg·kg⁻¹ (>20-year-old orchards), while row crop fields measured 25.97 mg·kg⁻¹. In 2021, Solvita CO₂-Burst measurements at 15 cm were also significantly higher (P < 0.001) in orchards 1–4 years old and older than that of row crop fields (Table 1). Orchard soils’ mean values ranged from 31.05 mg·kg⁻¹ to 49.65 mg·kg⁻¹, while row crop fields measured 21.52 mg·kg⁻¹. The CO₂-Burst values were significantly lower (P < 0.001) in samples taken at 91 cm (Table 2). There were no significant differences observed between orchards and row crop fields at this depth. The Solvita CO₂-Burst test has been used to evaluate soil CO₂ respiration in various crops (Chahal and Van Erd, 2018; Goupil and Nkongolo, 2014; Moore et al., 2019; Sadeghpour et al., 2016); however, there is little to no information regarding the use of this test to measure soil respiration in pecan orchard systems. The soil data from this study demonstrate that soil respiration is significantly enhanced in pecan orchards in this region when compared with conventionally farmed row crop fields. This increase in soil respiration could be due to an increase in total microbes present in the soil or to increased activity of the microbes present as a result of higher active carbon levels. We found that active carbon and CO₂-Burst measurements were positively correlated (r = 0.65) and that active carbon accounted for 42% of the variation in Solvita CO₂-Burst measurements (R² = 0.42) (Fig. 3), which agrees with the findings of Bongiorno et al. (2019). Orchard soils also contained larger amounts of organic matter, which would allow for the increase of microbial diversity and microbial activity (Cotrufo et al., 2013). We found CO₂-Burst values to be positively correlated (r = 0.64) with SOM, indicating that soil respiration increases as more organic matter is present in the soil (Fig. 4). SOM accounted for 40% of the variation (R² = 0.40) in Solvita CO₂-Burst.

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The Solvita SLAN test was used to measure the amount of potential plant available organic nitrogen in the soil. In 2020, Solvita SLAN was significantly higher (P < 0.001) in orchards 5 years and older than that of row crop fields (Table 1). Orchards 5–10, 11–20, and >20 years old measured 75.81 mg·kg⁻¹, 82.15 mg·kg⁻¹, and 134.19 mg·kg⁻¹, respectively, whereas row crop fields measured 57.42 mg·kg⁻¹. Solvita SLAN measurements were significantly higher (P < 0.001) in orchards >20 years old compared with that of row crop fields in 2021 at the 15 cm depth (Table 1). Orchards >20 years old measured 75.88 mg·kg⁻¹, whereas row crop fields measured 43.70 mg·kg⁻¹. Solvita SLAN values were significantly lower (P < 0.001) in samples taken at 91 cm (Table 2). There were no significant differences between orchards and row crop fields at this depth. Increasing SLAN concentrations indicate a larger pool of amino N present in pecan orchards. This increase of plant available organic nitrogen is tied to the increase in the stable humus portion of organic matter (Brinton, 2019a; Kelley and Stevenson, 1995). Larger pools of amino N present may also indicate more nitrogen mineralization taking place in orchard soils. An increase in available N reserves translates well to the enhancement of soil health and fertility in pecan orchards.

Soil pH was significantly higher (P < 0.001) in pecan orchard soils than that of row crop fields in 2020 and 2021 at 15 cm (Table 1). At this depth, the mean soil pH values averaged 6.4 and 6.5 for all orchards in 2020 and 2021, respectively, whereas mean soil pH of row crop fields averaged 5.9 each year. Soil pH is likely buffered in orchards by lack of cultivation and the increase in SOM (Helling et al., 1964). Samples taken at 91 cm in 2021 exhibit significantly lower (P < 0.001) soil pH values; however, we observed no significant differences between the soil pH of orchards and row crop fields at this depth (Table 2). Soils of the southeastern Coastal Plain are inherently acidic. The growth of pecans can be sensitive to soil pH (Wells, 2009), as a soil with low pH can negatively affect the tree’s roots by restricting their growth (White et al., 1982). Raising soil pH with liming materials is common practice in pecan orchards throughout the Southeast to ensure the availability of essential nutrients. Generally, maintaining a soil pH of 6.0–6.5 is recommended for pecan orchards in this region (Wells, 2017a).

CEC was significantly higher (P < 0.001) in orchards 11 years and older than that of row crops in 2020 (Table 1). Orchards 11–20 and >20 years old measured 6.37 meq/100 g and 9.26 meq/100 g, respectively, whereas row crop fields measured 4.88 meq/100 g. In 2021, CEC was also significantly higher (P < 0.001) in orchards 11 years and older compared with that of row crop fields at 15 cm (Table 1). Orchards 11–20 and >20 years old measured 5.89 meq/100 g and 8.61 meq/100 g, respectively, whereas row crop fields measured 4.59 meq/100 g. Soil samples taken at 91 cm indicate that CEC decreased slightly with depth. We observed no significant differences in CEC values between orchards and row crop fields at this depth (Table 2). Soils throughout the southeastern coastal plain region have an average CEC of ∼6 meq/100 g (Sonon et al., 2017). The data observed here indicate that over time, pecan orchard soils exceed average CEC values for the coastal plain region. The soils sampled across all sites have similar soil textures, so increases observed in CEC are likely partially due to the increase of organic matter content in pecan orchard soils. Ramos et al. (2018) found that soil CEC was reduced considerably in soils absent of organic matter. Soil pH also plays a considerable role in CEC. CEC generally increases with increasing soil pH (Sonon et al., 2017). Pecan orchard soils exhibited higher soil pH values, likely contributing to the enhanced CEC of these soils. Saikh et al. (1998) suggest that cultivation tends to reduce a soil’s CEC, which helps to explain the lower CEC values obtained from row crop fields.

Total N levels were significantly higher (P < 0.05) in orchards 11–20 years old compared with row crop fields in 2020 (Table 1). Orchards 11–20 years old measured 0.31%, while row crop fields measured 0.22%. In 2021, total N levels were significantly higher (P < 0.05) in orchards 5–10 and >20 years old compared with row crop fields at 15 cm (Table 1). Orchards 5–10 and >20 years old measured 0.22% and 0.23%, respectively, whereas row crop fields measured 0.17%. Measurements taken from samples at 91 cm revealed that mean values for total nitrogen were slightly higher than the values obtained at 15 cm in 2021 (Table 2). We observed no significant differences in total nitrogen values between orchards and row crop fields at 91 cm; however, the data may indicate nitrate leaching through the soil profile in both pecan orchards and row crop fields. The total N levels we observed at this depth may be due to the increase in rainfall in 2021, as nitrate is easily leached through the soil profile due to rainfall and irrigation (Wang et al., 2015). The higher levels of total nitrogen observed in orchard soils at 0–15 cm could be due to the amount of fertilizer N applied to pecans throughout the year. According to Wells (2017b), mature pecan trees require 34–68 kg of nitrogen each growing season. Lower levels of total nitrogen observed in row crop fields may be attributed to the degradation of organic matter and soil structure after cultivation (Emiru and Gebrekidan, 2013).

We observed higher levels of organic matter and microbial activity in orchard soils, which also contribute to increased soil N levels. The fact that total N levels in some orchard soils were higher than that of row crops at 0–15 cm but similar to that of row crops at 91 cm indicates that tree roots are likely removing nitrogen from the soil profile. This is supported by Allen et al. (2004), who suggest that pecan tree roots were able to capture N in a cotton-pecan alley-cropping system, resulting in lower rates of leaching below the root zone.

Aggregate stability in orchards >20 years old was significantly higher (P < 0.05) than in row crop fields in 2020 (Table 1). Pecan orchards >20 years old exhibited 7.05% aggregate stability while row crop fields only 5.16%. In 2021, aggregate stability was significantly higher (P < 0.05) in orchards 1–4 and 11–20 years old compared with row crop fields at 15 cm (Table 1). Orchards from the 1–4 and 11–20-year-old group measured 8.90% and 9.90% aggregate stability, respectively, whereas row crop fields measured 6.79%. Data from samples taken at 91 cm demonstrate that aggregate stability decreased slightly with depth; however, there were no significant differences between orchards and row crop fields at this depth (Table 2). The improved aggregate stability of pecan orchard soils is likely due to the significant increases in the amount of organic matter present. Increasing levels of organic matter help to improve the formation of stable soil aggregates. This increase in aggregate stability could also have been due to the significant increase in microbial activity we observed in pecan orchards. Microbial activity in soil has shown the ability to increase the formation of soil aggregates (Idowu et al., 2017; Six et al., 2004). Good aggregate stability improves pore space, which can increase water and air infiltration as well as allow for deeper root exploration (Amezketa, 1999; Breker, 2021b; Kemper and Rosenau, 1986). Faster water infiltration coupled with better water retention can help to reduce runoff and erosion throughout pecan orchards. Good aggregate stability also promotes a better habitat for microorganisms (Breker, 2021b).

Bulk density measurements were nearly identical across all sites in 2020. We observed no significant difference in bulk density between orchards and row crop fields (Table 1). Mean bulk density values averaged 1.34 g/cm³ for all orchards, whereas row crop fields measured 1.40 g/cm³. Soil samples taken in 2021 yielded similar results. No significant differences in bulk density were observed between soils of pecan orchards and row crop fields at 15 cm (Table 1). Bulk density measurements from samples taken at 91 cm were significantly lower (P < 0.001) than the values obtained from samples at 15 cm; however, we observed no significant differences between orchards and row crop fields at this depth (Table 2). The bulk density measurements from samples taken at 91 cm are much lower than expected, as bulk density generally increases with soil depth. Other soil quality indicators analyzed in this study that correlate with bulk density indicate that the values obtained at 91 cm should have been higher. This may be due to a sampling error at this depth. Higher bulk density values near the soils surface could also indicate some compaction, which could likely be attributed to equipment traffic due to management practices. Sandier soils generally have higher bulk densities than clay or silt soils because they have less porosity (Hao et al., 2008). The average range of bulk densities for sandy soils is 1.2–1.8 g/cm³ (Chaudhari et al., 2013). The ideal bulk density for unrestricted root growth in sandy soils throughout this region is ≤1.60 g/cm³ (U.S. Department of Agriculture Natural Resources Conservation Service, 2019). The bulk density measurements we obtained align with these thresholds, however, we saw no significant enhancement of bulk density in pecan orchards compared with row crop fields.

In 2020, we observed no significant differences in soil porosity between pecan orchards and row crop fields (Table 1). Mean soil porosity values of pecan orchards ranged from 56.18% to 63.74%, whereas row crop fields measured 59.99%. Soil samples taken in 2021 yielded similar results, in which no significant differences were observed between orchards and row crop fields at 15 cm (Table 1). Mean soil porosity values of pecan orchards ranged from 61.87% to 66.27%, while row crop fields measured 62.88%. Porosity measurements from samples taken at 91 cm were significantly lower (P < 0.001) than porosity values obtained from samples at 15 cm; however, we observed no significant differences between orchards and row crop fields at this depth (Table 2). At 91 cm, mean porosity values for pecan orchards ranged from 39.24% to 46.83%, whereas row crop fields measured 43.80%. Soil porosity values obtained from 0 to 15 cm are higher than average as Hazelton and Murphy (2016) suggest that the porosity of a typical agricultural soil is about 47%. Hao et al. (2008) reported that the porosity of sandy soils generally ranges from 35% to 50%, but can extend up to 60% with some finer-textured sands. Porosity and bulk density values were consistent across all sites in 2020 and 2021 (0–15 cm); therefore, we did not observe any significant improvements in pecan orchards compared with row crop fields. These results are likely due to the uniformity of soil texture throughout the sampled areas in this region.

Conclusions

Based on the soil quality indicators measured here, pecan orchards under commercial management have significantly higher levels of improved soil health at 15 cm compared with row crop fields in the southeastern Coastal Plain. The management practices of these two different cropping systems have considerable influence on improving soil quality. The annual cultivation of row crop land aids in soil degradation and reduced biological activity, while the largely undisturbed soils of pecan orchards allow for the perennial accumulation of organic matter. We observed significantly higher levels of SOM in pecan orchards of various ages in both years of the study, with the highest levels occurring in the oldest orchard age range. Active carbon and Solvita CO₂-Burst measurements suggest that pecan orchards exhibit significantly higher rates of microbial activity and soil respiration. Indicators of soil microbial activity such as active carbon and Solvita CO₂-Burst were strongly correlated with SOM, which explained much of the variation observed in these measurements of soil microbial activity. Although we observed no differences in bulk density and porosity between pecan orchards and row crop fields, aggregate stability did significantly increase in orchard soils with orchard age. Pecan orchards exhibited a much higher CEC with age when compared with row crop fields. The higher Solvita SLAN and total N measurements we observed in pecan orchards are associated with an increase in soil fertility. These results also suggest that pecan roots are capable of removing excess nitrogen from the soil profile, reducing nitrate leaching. Results from the selected soil quality indicators provide evidence that the soil quality of land previously used for conventional row cropping systems in the southeastern Coastal Plain can be significantly improved by orchard establishment and can improve with orchard age over time.