Soil Change after Three Decades of Conventional Till

Soil Change after Three Decades of Conventional Till, No-Till, and Forest Succession in the Piedmont of Georgia, USA S. Devine 1, D. Markewitz 1, P. Hendrix 2, 3, and D. Radcliffe 3 1 D. B. Warnell School of Forestry and Natural Resources, 2 Odum School of Ecology, and 3 Department of Crop and Soil Sciences, University of Georgia, Athens, GA, USA 1938 Abstract Land use affects soil properties and functions, which has important implications for biogeochemical cycles. The Horseshoe Bend (HSB) tillage experiment is a long-term investigation of no-tillage (NT) and conventional tillage (CT) agroecosystems begun in 1978 in the piedmont of Georgia, USA (Fig. 1 and 2). Nutrient cycling studies at HSB in the 1980 s focused on how tillage alters nutrient retention, showing that NT soils may conserve more N, K, Ca, and Mg. In the 1990 s, the focus shifted to differences in soil organic matter (SOM) dynamics in the upper 15 cm, showing that greater soil C contents from 0 -5 cm in NT soils may be related to physical protection of SOM within soil aggregates. In this work, we sought to re-examine these research questions by emphasizing a full profile perspective (0 -2 m) and by comparing the tillage treatments to adjacent forest succession (FS) plots. We measured total C and N, exchangeable nutrients, and p. H from 0 to 2 m and quantified soil aggregate stability in the upper 15 cm. FS soils have greater soil C concentrations only in the 0 -5 cm layer (Fig. 3 a) that is associated with greater aggregate stability (Table 2). Soil C contents, however, do not differ between NT and FS surface soils, and FS has no significant increases in deeper soil C contents (Fig. 3 a). Overall, C contents of both the NT and FS profiles are greater than the CT profile (Table 3). Nutrient chemistry also differs through the profiles with CT-NT soils having higher P to 30 cm, K to 100 cm, Ca to 200 cm, and Mg to 50 cm (Figs. 3 c-f). NT and CT soils also have greater effective cation exchange capacity (ECEC) through 2 m (Fig. 3 h). In contrast, FS soils have greater acidity through at least 2 m (Fig 3 i); exchangeable acidity in FS decreases from a peak of 1 cmolc/kg at 5 -15 cm to 0. 02 cmolc/kg from 50 -200 cm (Fig 3 g). This low exchangeable acidity at depth in FS suggests a presence of residual lime from previous pasture management, though differences in the exchangeable Ca content of the FS profile relative to the CTNT profiles greatly exceed Ca inputs as lime over the course of the experiment (Table 3). This Ca content difference between FS and CTNT suggests that the FS profile has experienced a relatively dramatic loss of Ca from some combination of uptake by roots and solution leaching. Solution leaching would have been enhanced by loss of p. H dependent, negatively charged sites as a result of gradual acidification under FS. Finally, while a full profile analysis reveals greater nutrient availability under NT than under CT (Table 3), these differences are relatively minor compared to the more dramatic soil change that has occurred under FS over three decades. 1967 1999 Results • Soil C concentration differs significantly only in the 0 -5 cm layer: Forest Succession (FS) > No-Till (NT) > Conventional Till (CT) (Figure 3 a). Soil C content in 0 -15 cm is 24. 4 in NT, 23. 7 in FS, and 18. 8 in CT. Soil C content from 0 -2 m is significantly greater in FS and NT compared to CT (Table 3). • Aggregate stability as measured by the structural stability index increases from CT to NT to FS in both A horizon depths (Table 2). Forest Succession Plot Tillage Experiment Area • Compared to CT-NT, FS soil profile has significantly lower concentrations of P to 30 cm (Fig. 3 c), K to 1 m (Fig. 3 d), Ca to 2 m (Fig. 3 e), and Mg to 50 cm (Fig. 3 f). • FS soil p. H and ECEC are significantly lower than CT-NT from the surface to 2 m depth (Figs. 3 h-i), despite the fact that agricultural plots were last limed in 1980. Figure 2. Horseshoe Bend (HSB) was managed as pasture by the UGA Dairy from the 1930 s to 1966 with periodic applications of dolomitic limestone to both the current tillage experiment area and forest succession plots. HSB was acquired by the Odum School of Ecology in 1966 and natural succession was then allowed to proceed for twelve years until 1978 when successional vegetation was cleared within the area of the tillage experiment. Since then, forest succession has continued for another three decades adjacent to the tillage experiment. • Differences in exchangeable Ca contents of FS relative to CT-NT profiles (0 -2 m) greatly exceed the quantity applied as lime, while differences in extractable P of FS relative to CT-NT profiles is an order of magnitude less than the estimated P fertilization (Table 3). • NT profile nutrient content exceeds CT profile (0 -2 m) by: 16% N, 28% P, 13% Ca, and 15% Mg (0 -1 m), though differences are not statistically significant (Figs. 3 b-f, Table 3). Table 2. Aggregate characteristics for dry and wet sieving. Stability index is a ratio of Wet-Sieved Aggregate Mean Weight Diameter (MWD) to Dry-Sieved Aggregate MWD. Research Questions (1) Have three decades of continuous conventional till, no-till, and forest succession led to differences in soil carbon contents over a 2 m profile? (2) Hypothesis 1: Soil carbon contents (0 -2 m) will increase in the order of Forest Succession > No-till = Conventional Till. Greater soil C accumulation in the upper horizon of no-till will be compensated by greater soil C accumulation in conventional till below the surface horizon. (2) Have forest succession and the tillage treatments led to differences in soil chemical properties and nutrient contents, especially below the surface horizon? Hypothesis 2: (a) Lime and fertilizer applications, rather than forest succession, will largely explain an expected divergence in soil chemical properties between the forest succession and tillage experiment soils. (b) Observations of lower solution concentrations of N, K, Ca, and Mg at 60 cm depth in the no-till soils in the early 1980 s are expected to be evident today as greater nutrient availability to 2 m under no-till. a b c Table 3. Mean element contents for whole soil profiles (0 -2 m) in 2007 and fertilizer inputs applied to conventional till and no-till soils since the beginning of the experiment (1978 -2007). Table 1. Physical properties of the A horizon sampled in 2006. Soil Taxonomic class: fine loamy, siliceous, thermic, Rhodic Kanhapludult Forest Succession d Conventional Till e f No-till Notes: Treatments that a share a letter for a particular nutrient are not significantly different using Tukey’s HSD (α=0. 05, n=4). Bulk density assumed to be 1. 5 g cm-3 from 0. 15 to 2 m. Sum of Mg is only to 1 m, because of great variability below 1 m (Fig. 3 f) that suggests differences in mineralogy due to temporal variability in alluvial deposition events. ANOVA not run for total profile P due to missing data. Methods Figure 1. HSB Tillage Experiment on an old alluvial terrace in Winter 2007. Tillage consists of moldboard plowing and disking before planting of summer and winter crops. • Four forest succession plots dominated by Quercus spp. were established in 2006 adjacent to the tillage experiment begun in 1978 (Fig 2 a-c); n=4 for each treatment. • Soil samples in 7 depth classes: 0 -5, 5 -15, 15 -30, 30 -50, 50 -100, 100 -150, and 150 -200 cm (Table 1). • Total C and N determined by pulverization and dry combustion, Mehlich I extracts analyzed for P, Ca, Mg, and K; 1 M KCl extracts titrated for exchangeable acidity; ECEC determined by sum of cations (K + Ca + Mg + Ex. Ac); p. H in 0. 01 M Ca. Cl 2 • 0 -5 and 5 -15 cm soils were fractionated by both wet and dry sieving into <0. 053, 0. 053 -0. 25, 0. 25 -2, and >2 mm aggregate size classes. • Differences in each soil property were tested by one-way ANOVA with land use as the main effect (n=4) in each depth class, using Tukey’s HSD (α=0. 05) for means separation Conclusions • Contrary to Hypothesis 1, three decades of forest succession did not lead to greater C contents relative to the no-till soil. However, both forest succession and no-till soils have greater overall C contents than the conventional till soil over the entire 02 m profile (Table 3) with 60% of this additional content occurring from 0 -15 cm. g Figures 3 a-i. Data h i points are means ± 1 SE (n=4). Most dramatic changes have occurred with respect to exchangeable Ca, acidity, and p. H in the forest succession soils to a depth of 2 m. • Contrary to hypothesis 2 a, there is evidence that forest succession, rather than liming, is largely responsible for the observed differences in soil Ca and acidity. Current Ca availability in the CT-NT profiles exceeds that in the FS profile by 8 times the Ca applied in lime to the CT-NT treatments (Table 3). • In concurrence with hypothesis 2 b, element totals from 0 -2 m (Table 3) support earlier nutrient cycling studies at HSB that showed no-till soils lose less N, K, Ca, and Mg through leaching relative to conventional till. These treatment differences are minor compared to the soil change that has occurred under forest succession.