Agroecological Intensification of Warm-season Pastures for Improved Productivity and Quality and Ecosystem Services

The aim of this study was to evaluate the performance of warm-season perennial pastures in terms of forage production and quality along an AEI gradient of increasingly intensified management as well as the impact of AEI on soil organic matter.

Objective 1: Forage productivity and quality

The first objective of this study was to examine the quantity, evenness, and quality of forage production across an AEI gradient. Annual forage accumulation varied by year, with less forage accumulated in 2022 compared with 2021 (Figure 1; Table 1). In year 1 of the study, a trend towards increasing forage accumulation with increasing management intensification was observed. Inclusion of the warm-season legume rhizoma peanut (RP) had a greater impact on increasing total annual forage than did applying warm-season N fertilization alone and in combination with overseeding cool-season grasses + legumes (Table 1). Rhizoma peanut enhanced the rate of warm-season forage regrowth between defoliation events in May-October in 2021 and from June-September in 2022 (Figure 2). Warm-season N fertilization and over-seeding cool-season forages improved the evenness of forage accumulation in BG during the second year of the study (Figure 3). Over-seeding cool-season grasses + legumes and applying 40 kg N ha^-1 plus overseeding cool-season grasses had similar effects on promoting evenness compared with unfertilized BG. In 2022, all treatments with BGRP displayed greater evenness of forage production than unfertilized BG.

Table 1. Intercept and varying slopes from the model for forage accumulation. Coefficients represent deviations of each fixed effect (AEI gradient) from the baseline bahiagrass (BG) up to inclusion of rhizoma peanut (BGRP), warm-season N fertilization (40 kg N/ha), and cool-season grasses (CG) plus cool-season grasses+legumes (CGL). Negative values represent comparatively lower forage accumulation and positive values comparatively higher forage accumulation.

Forage nutritive value

We examined the protein content and digestibility of forages to evaluate the effect of different management practices on forage quality. Nitrogen serves as a proxy for crude protein, since it is essential in the amino acids that build proteins. Rhizoma peanut inclusion enhanced the forage N concentration compared with unfertilized bahiagrass, even in the cool-season when RP was not present in the botanical composition due to dormancy (Figure 4). The improved forage quality may be the result of a greater rate of N mineralization in the presence of RP (Kohmann et al., 2019). Nitrogen concentration of cool-season forages ranged from 14.15 g kg^-1 for BG40CG (5 on the management intensification gradient) to 19.00 g kg^-1 for BGRP40CGL (12 on the management intensification gradient). Warm-season forage N concentrations were between 14.09 g kg^-1 for unfertilized BG to 18.61 g kg^-1 for BGRPCG (8 on the management intensification gradient). These values are slightly lower than those observed for pure RP stands in the Florida Panhandle (Dubeux et al., 2017), likely due to the inclusion of grasses, which are of lower nutritive value than legumes in general.

In vitro organic matter digestibility was slightly higher on average for cool-season forages than for warm-season forages (Figure 5). Averaged across treatments, IVOMD was 695.64±84.42 g kg^-1 for cool-season forage and 629.69±49.98 g kg^-1 for warm-season forage. The slightly higher IVOMD values for cool-season forages are likely due to differences in the leaf tissue anatomy between C₃ and C₄ plants. Bahiagrass, a warm-season (C₄) grass, has more complex carbohydrates in the cell wall than found in broadleaf and cool-season (C₃) plants like RP, rye, oat, ryegrass, and clovers (Coleman et al., 2004). Consequently, C₄ forage tends to be less digestible than C₃ forage. Our range for cool-season forages was slightly lower than the averages observed for rye-oat, rye-oat-ryegrass, rye-oat-crimson clover, rye-oat-ryegrass-crimson clover, rye-oat-red clover, and rye-oat-ryegrass-red clover in 1996 and 1997 (706 and 736 g kg^-1, respectively; Fonatelli et al., 2000). The lower IVOMD of cool-season forages that we observed might be explained by less N fertilization applied in our study (40 kg N ha^-1) compared with Fonatelli et al. (2000; 120-160 kg N ha^-1).

Objective 2: Belowground impacts on soil microbial biomass C and SOM

Our second objective was to quantify below-ground root and rhizome production and link them to changes in microbial biomass and SOM at each level of AEI. Soil microbial biomass C ranged from 0.03-0.06 mg C g soil^-1 in the cool season and 0.02-0.05 mg C g soil^-1 in the warm season. A decline in soil microbial biomass C was observed as C in roots and rhizomes increased during the cool season but the trend changed directions in the warm season (Figure 6). At lower levels of root and rhizome C, BGRP enhanced soil microbial biomass C. Meanwhile the application of 40 kg N ha^-1 in May 2022 enhanced soil microbial biomass relative to 0 kg N ha^-1 although the two had similar increasing slopes.

Greater variation was observed in the C and N concentrations in the coarse sand fraction than in the medium to fine soil fraction at the beginning of the experiment but variation in this fraction declined after two years of applying the experimental treatments (Figure 5a and 5b). The δ¹³C signature was more negative (more C4) in the coarse sand fraction than in the medium to fine fraction, suggesting greater contributions from BG (Figure 5c). However, after two years of applying the experimental treatments, the δ¹³C signature in the coarse sand fraction slightly declined, suggesting greater contributions of C₃ materials (e.g. RP and cool-season forages). The δ¹⁵N signature of medium to fine sand was enriched from the beginning to the end of the experiment, suggesting that a low rate of warm-season N fertilization enhanced N in these soils since N isotopic signatures near zero suggest active biological N fixation (Dubeux et al., 2017), which would have contributed more to the δ¹⁵N signature in the absence of N fertilization. 

The effect of the experimental treatments on soil fertility was to increase the spread of values observed. Higher pH, Ca, and Mg had a positive impacts on net primary productivity (NPP, Figure 8a, 8d, 8e). The range of P and K expanded by the end of the study (Figure 8b, 8c). Copper was enriched from 0.2-0.6 mg kg^-1 in year 1 of the study to 0.4-0.8 mg kg^-1 (Figure 8g). The range of manganese and zinc both increased slightly from the start to the end of the project, which corresponding gains in NPP (Figure 8h, 8i).

Objective 3: Impacts of the proportion of rhizoma peanut and soil moisture

The proportion of RP increased from 2021 to 2022 (Figure 9). Rhizoma peanut enhanced forage accumulation in 2021 compared with sole bahiagrass. As the proportion of RP increased in 2021, greater forage accumulation was observed with 0 kg N ha^-1 than given 40 kg N ha^-1. However, greater forage accumulation was observed at all levels of proportion RP for 40 kg N ha^-1 in 2022 than for 0 kg ha^-1.   

Soil volumetric water content was greater on average in 2021 than in 2022 (Figure 11).  Cumulative precipitation during the warm season was approximately 220 mm greater in year 1 of the study compared with year 2 (Table 2). The proportion of RP as a component of the forage biomass had a greater range in 2022 (Figure 11). This is likely due to superior performance of RP under limited precipitation than BG (Williams, 1994). Reduced precipitation in June and July 2022, when evaporative pressures are high, may have stunted BG growth that year due to limited soil water availability. Thus, RP constituted a greater proportion of the total forage biomass in the drier of the two years of the study.