This experiment was conducted in 2011 and 2012 at the Texas A&M AgriLife Research and Extension Center near Overton, TX (32˚17'N, 94˚58'W). Land was previously managed as a fertilized (split-applied 145-45-45 kg∙ha⁻¹ N-P₂O₅-K₂O annually), permanent bermudagrass (Cynodon dactylon [L.] Pers.) pasture for 10 years prior to cultivation in 2009 in preparation of other agronomic research. Plots for this study were located on a Lilbert loamy fine sand (Loamy, siliceous, semiactive, thermic Arenic Plinthic Paleudult) with an initial pH of 5.12 and 4.26 at 0 to 15 and 15 to 30 cm depths, respectively. Based on initial soil samples taken on August 14, 2009, soil was limed on September 2, 2009 with 9.2 t∙ha⁻¹ of lime (ECCE 100), which raised pH levels to 6.93 (0 to 15 cm) seven months later. In addition, 337 kg∙ha⁻¹ of 0-60-60 N-P₂O₅-K₂O was broadcast applied on November 5, 2009 to meet crop nutrient demands. Weather was unseasonably hot and with extreme drought conditions in 2011 followed by more moderate temperatures and moisture in 2012 (Figure 1 and Figure 2).

Two separate experiments were conducted; one to evaluate the impact of WSL and one to evaluate the impact of CSL on SOC, soil N, soil moisture and green manure potential. Each experimental design was a randomized complete block design with four replicates. Warm-season legumes and CSL were randomly planted within replications. Individual plot size was 1.5 × 6.0 m. Because WSL were planted more than five months prior to CSL, they received an additional winter rye forage/winter fallow treatment while CSL were growing through the winter months. All plots then were planted to a sorghum-sudangrass crop to capture leguminous N contributions. All crops were planted using a Hege double-disk, single-cone small plot planter on 18-cm row centers.

Legume species and cultivars were chosen for this study to represent a range of

commercially available products that were well-adapted to local soils and climate. Nine WSL cultivars consisting of ‘Iron-and-Clay’ and ‘Combine’ cowpea (Vigna unguiculata [Walp]), ‘Laredo’ and ‘Vernal’ soybean (Glycine max L.[ Merr.]), ‘Rio Verde’ lablab (Lablab purpureus L. [Sweet]), ‘Kobe’ lespedeza (Kummerowia striata [Thunb.]), mungbean (Vigna radiata L. [Wilczek]), alyceclover (Alysicarpus vaginalis L. DC), pinto bean (Phaseolus vulgaris L.), and a summer fallow treatment were randomly planted within each block in 1.5 × 6.0 m plots in the spring (May 24, 2011) and incorporated as green manure in the fall (August 19, 2011). Eight CSLs including ‘Dixie’ crimson clover (Trifolium incarnatum L.), ‘Apache’ and ‘Blackhawk’ arrowleaf clovers (Trifolium vesiculosum [Savi.]), ‘R18’ rose clover (Trifolium hirtum A.), Hairy vetch (Vicia villosa [Roth]), ‘Whistler’ and common Austrian winter pea (AWP) (Pisum sativum spp. Arvense L.), and caleypea (Lathyrus hirsutus L.) were then planted in the fallow plots in the fall (November 10, 2011). All legumes were planted at the following standard [21] seeding rates: 73 kg∙ha⁻¹ for pinto bean; 56 kg∙ha⁻¹ for cowpea, soybean, and caleypea; 39 kg∙ha⁻¹ for AWP; 34 kg∙ha⁻¹ for mungbean and lespedeza; 28 kg∙ha⁻¹ for lablab and vetch; 22 kg∙ha⁻¹ for crimson clover; 18 kg∙ha⁻¹ for rose clover; 17 kg∙ha⁻¹ for alyceclover; and 11 kg∙ha⁻¹ for arrowleaf clovers. Following rototilling, a roller-packer was used to create a firm seed bed for rye and sorghum-sudangrass planting.

Prior to rototilling under the WSL green manure crops in the fall, a 0.09 m² quadrat of shoot biomass from each plot was hand clipped and roots removed by shovel (approximately 30 cm depth) before drying at 60˚C to determine dry matter (DM) accumulation and C and N concentrations in the biomass. To compare legume impact on soil moisture, gravimetric moisture samples were taken in the fall (August 24, 2011) and spring (April 9, 2012). A single one meter (32 mm diameter) deep soil core was taken congruently with biomass samples from each plot in the fall prior to incorporation, while a composite sample was made from three (22 mm diameter) 60 cm deep subsamples in the spring.

Nylon degradation bags were constructed to bury aboveground legume biomass in plots and later bags were recovered at various dates to estimate N mineralization and remaining C in the soil throughout each growing season [22] . Because of small plot sizes and limited available biomass for decomposition bags, a 1.0 kg bulk sample of each legume was harvested from outside each plot area and sorted into stems, leaves, and blossoms/pods. Blossoms were separated out for Dixie and Rose clovers, and pods were only present on mungbean and Laredo soybean at the time of incorporation. A standard ratio for each legume was calculated between leaf, stem, and blossom/pod and used to fill 10 × 18 cm nylon mesh (250 µm) bags, which were buried within plots immediately after green manure incorporation. Five bags were buried per plot at the plow depth (15 cm) and recovered 5, 10, 16, 27, and 34 weeks after burial for WSLs and 2, 4, 6,10, and 23 weeks after burial for CSLs. The HOBO Pendent® temperature sensors/data loggers (Onset Computer Corp., 2012) were installed in nine selected plots at the time of incorporation and recorded hourly soil temperature throughout the fall, winter, and spring. Recovered biomass from the bags was dried at 60˚C a nd weighed to determine remaining residue. Dried samples were then ground to 1 mm particle size and analyzed for C and N concentrations using combustion analysis. Dried plant samples (275 mg) were analyzed with an Elementar Analyzer-vario MAX CNS unit (Elementar Analysensysteme GmbH, Hanau, Germany) [22] [23] .

After incorporation by rototilling, each WSL treatment was followed by winter fallow or ‘Elbon’ winter cereal rye (112 kg∙ha⁻¹ seeding rate) planted on November 9, 2011. Five rye forage harvests were cut on November 30, January 18, February 6, March 1, and April 10 during the 2011-2012 winter using a mechanical harvester and subsampled for moisture content to estimate total DM yield. Following the final rye forage harvest and rototilling of the winter rye stubble and CSLs, plots were rototilled and rolled to prepare a firm seedbed. Sorghum-sudangrass summer forage was planted at 39 kg∙ha⁻¹ over all treatments on May 9, 2012. Stand emergence was taken on May 25, 2012 by counting plants in a one-meter length section of the middle two plot rows. Sorghum-sudangrass forage color was ranked on a 1 (yellow) to 5 (dark green) scale on June 12, 2012. Three forage harvests were taken on June 25, August 15, and September 25, 2012 using a mechanical harvester and subsampled for moisture to estimate DM yield. Combustion analysis was done on rye forage samples to determine treatment effects on N concentrations of forage and estimate total N accumulation removed by the rye crop. Three soil cores were taken per plot and split into 0 to 15, 15 to 30, and 30 to 60 cm depths after each crop termination. Soil samples were dried at 60^oC and ground (<840 um), and soil C and N levels were measured using combustion analysis as described for plant samples except using 1000 mg as recommended [23]. A fizz test using 10% HCl was negative and confirmed inorganic C was below detectable levels.

All crop yield and quality data were analyzed with PROC GLM model using SAS 9.2 [24] . All treatments were considered fixed effects, and replication was designated as a random effect. Fall soil data, rye forage yield and N data, and legume yield data were analyzed as one-way ANOVAs; whereas, LSD (P > 0.05) was used for mean separation of CSL data and WSL data separately. Spring soil data and sorghum-sudangrass forage data were separated and analyzed as a one-way ANOVA for CSL treatment effects and as a two-way ANOVA for WSL and rye treatment effects.

Because legume green manures add substantial amounts of fixed C and N to the soil when incorporated, soil samples were taken at each crop termination to determine treatment effects on SOC and soil N. Prior to WSL green manure incorporation, soil analysis revealed higher SOC and soil N at both the 0 to 15 cm (P < 0.01) and 15 to 30 cm (P < 0.01) depths under Combine cowpea, Iron-and-Clay cowpea, and mungbean, which were the highest biomass producers as well. On average, these legumes increased SOC by 41% and 61% at 0 to 15 and 15 to 30 cm depths, respectively, compared to fallow plots (Figure 5). By

April, these effects were no longer present in the upper two soil layers, but were instead significant (SOC P = 0.06; soil N P = 0.10) for Combine and Iron-and-Clay cowpea at 30 to 60 cm. Overall, SOC and soil N decreased at this depth from August through April (fallow decreased 38%); however, Combine and Iron-and-Clay cowpea cultivars appeared to minimize losses due to increased N and C inputs. These inputs may have come from downward movement of N and C in the soil due to leaching of NO 3 − and soluble SOC. If root biomass production was responsible, then effects should have been present in August.

Cool season legumes had no effect on SOC or soil N at the 0 to 15 cm or 30 to 60 cm depths; however, a winter crop treatment resulted in a significant increase in soil C and N at 15 to 30 cm. Surprisingly, winter rye had the highest significant SOC (5.0 g∙kg⁻¹; P = 0.03) and soil N (0.5 g∙kg⁻¹; P = 0.04) at this depth along with AWP (Table 9). Winter rye should have taken up soil N through the roots, incorporated it into aboveground biomass, and been removed with each forage cutting, thereby reducing overall soil N. It is unlikely that rye root production and mineralization after crop termination would have been responsible for such a response. However, visual assessment of sorghum-sudangrass indicated darker green biomass under winter rye compared to winter fallow in June.

Summer legumes had little effect on rye forage yield. In the first cut, rye yield following summer fallow (1.00 t DM ha⁻¹) was significantly higher (P = 0.08) than rye following mungbean (0.64 t DM ha⁻¹) (Table 10). The same was true for N concentrations in the rye biomass. Mungbean reduced (P = 0.03) N concentrations by 5% in rye forage during cut one compared to summer fallow (57.3 g∙kg⁻¹). Reduction in rye yield and N concentration was likely due to soil moisture depletion, since mungbean had the least amount of soil water available by the end of the summer, and the dry fall allowed for little recharge prior to rye planting.

There were no differences in yield or N concentrations among summer treatments in subsequent rye cuttings or overall yield (12.55 t DM ha⁻¹). Average rye yields for each cut were 0.81, 2.04, 0.99, 1.37, and 7.34 t DM ha⁻¹ sequentially. Increased moisture throughout the winter months should have led to an N response from the legume residue, since an average total of 376 kg N ha⁻¹ was removed in rye biomass by the last forage harvest. Mineralization of N from a large SOC pool already present in the field prior to the study may have provided enough available N to mask this effect. Visual assessment supported this idea as rye plots did not exhibit yellowing on unfertilized fallow plots, which is commonly associated with N deficiency. Soil analysis confirmed adequate levels of macro- and micronutrients prior to the study and suggest nutrient deficiencies are unlikely.

^†Different letters within column and Depth are significant at P = 0.05.

^†Different letters within column and legume treatment are significant at P = 0.05 level.

Similar to rye, sorghum-sudangrass was largely unaffected by summer legume green manures. Initially, Iron-and-Clay cowpea treatments had increased sorghum-sudangrass stand counts (9.8 plants linear m⁻¹) over Vernal soybean (8.1 plants linear m⁻¹) or mungbean (7.1 plants linear m⁻¹) (Table 11); however, this stand difference did not translate into differences in forage yields. On June 12, sorghum-sudangrass was darker green in color (P = 0.03) following rye compared to winter fallow, suggesting a nutrient contribution (Table 11). Rye roots may have mined nutrients from deeper in the soil profile and released them in the upper soil layers as root material was mineralized. Root exudates from rye could have also stimulated microbial activity and thus increased mineralization of SOC and release of additional nutrients.

Cut three had a summer crop × winter crop interaction (P = 0.02) where a summer fallow-winter rye rotation increased sorghum-sudangrass yield (9.84 t DM ha⁻¹) by 33% compared to a summer fallow-winter fallow rotation (7.38 t DM ha⁻¹) (Table 12). Following mungbean, winter rye decreased subsequent sorghum-sudangrass yield (9.71 t DM ha⁻¹) by 28% over winter fallow plots (7.01 t DM ha⁻¹). Increased sorghum-sudangrass forage yield following summer fallow-winter rye compared to summer fallow-winter fallow was surprising since rye utilized soil moisture and nutrients that were removed in forage biomass. Rye did contribute organic matter and likely some N through root production and mineralization, but amounts were much lower than N removed in aboveground forage. Yield decrease from mungbean-winter fallow to mungbean-winter rye was expected since crops generally experience a yield loss following another crop if additional fertilizer is not applied.

Cool-season legumes had minimal impact on sorghum-sudangrass. All CSLs, except for Blackhawk arrowleaf clover, increased (P < 0.01) the dark green color

^†Different letters within column are significant at P = 0.05 level.

^†Different letters within column are significant at P = 0.05 level.

of sorghum-sudangrass over winter rye or winter fallow on June 12, which is consistent with N fertilization of forage crops (Table 13). Cool-season legumes had no effect on forage yield of cuts one or two; however, sorghum-sudangrass following winter rye yielded higher (P = 0.05) than any of the legume treatments for cut three, except for hairy vetch, which was not different (Table 14).

Cut three had a summer crop x winter crop interaction (P = 0.02). In this case, winter fallow rotation preceded by mungbean was 32% higher for sorghum-sudangrass yield (9.71 t DM ha⁻¹) compared to winter fallow plots preceded by summer fallow (7.38 t DM ha⁻¹). Strangely, the opposite was true in plots that received winter rye as a winter treatment. Under this rotation, fallow (9.84 t DM ha⁻¹) and Vernal soybean (9.47 t DM ha⁻¹) treatments yielded approximately 26% more forage than Laredo soybean (7.23 t DM ha⁻¹) or mungbean (7.01 t DM ha⁻¹) treatments. Summer fallow-winter rye and Vernal soybean-winter rye rotations had very low or no N contributions compared to mungbean, which contributed 167 kg N ha⁻¹ to the soil the previous summer. Soil water usage by mungbean may have contributed to this yield depression as sorghum-sudangrass exhausted soil water supplies during the summer months. A winter rye crop may have prevented sufficient recharge of soil moisture over the winter months compared to winter fallow. Winter rye did not affect mungbean

^†Different letters within column are significant at P = 0.05 level.

^†Different letters within column are significant at P = 0.05 level.

residue decomposition. Thus, N mineralization and overall contribution of N to the soil by mungbean residue was the same regardless of winter treatment.