Compost application on rangeland in the semi-arid southwest for increased soil C storage and forage production

Polk’s Folly Farm is a small family run operation in the Eastern foothills of the Sandia Mountains (elevation 2095m). They specialize in heritage breed hogs, and also raise a small flock of laying hens, ducks, and a few head of cattle. The average temperature is 10.6C, with highs in July of 20.1C and lows in December of -0.9C, and average precipitation of 431 mm with 55% falling between July-October (NOAA 1991-2020). Soils are Silver and Witt very fine sandy loam at the surface (Web Soil Survey). This pasture was continuously and intensively grazed by horses from 1976 through the late 1990s. The pasture was fallowed until 2014 and mowed annually. In 2015 the pasture has been intermittently grazed by cattle using temporary electrical fencing to move herds ranging in 2-8 head through small paddocks. The degraded rangeland dominated by snakeweed (Gutierrezia sarothrae) and blue gramma (Bouteloua gracilis)

Sol Ranch is a restoration-focused cattle ranch that produces grassfed beef in northeastern New Mexico , where the plains meet the mountains (elevation 1890m). The average temperature is 10.0C, with highs in July of 20.4C and lows in December of 0.5C, and average precipitation of 400 mm with 63% falling between July-October (NOAA 1991-2020). Soils are Vegocito and Gallinas clay loam soils (Web Soil Survey). Before the 1950s, the land was used year-round for grazing sheep, after which it was used to graze cattle annually.  

Compost Production and Testing

Polk’s Folly Farm produced all compost used for this project. Primary feed stocks for the compost are food scraps and wood mulch. Additional feed stocks are horse, pig, and chicken manure, cardboard, and straw. About 25,000 lbs/week of expired fruits and vegetables are sourced from Road Runner Food Bank and deposited onto a bed of C-rich material, where it is physically mixed with swine manure. When a suitable mixture and moisture level are achieved, the compost is mounded in windrows. Windrows are temperature tested daily and turned or aerated with a skid steer as needed. A minimum of 131˚F is maintained for at least 3 days on each pile, most piles sustain temperatures of greater than 160˚F for more than 5 days, and are turned at least 5 times. When piles begin to cool they are consolidated and allowed to cure for a minimum of 30 days. 

Table 1. Compost characteristics from Colorado State University laboratory.

Experimental set-up

At each site, twelve 41.8 m2 plots (4.75 x 9.14 m) were established on flat, relatively uniform fields. Three plots were randomly assigned to be control plots, and three plots each received 0.64 cm, 1.3 cm, or 2.5 cm of compost which corresponds to approximately 0, 3.2, 6.4, 12.8 kg m2, respectively, which spans added amounts in previous studies (Kutos et al. 2022). Baseline monitoring (see responses below) was completed just prior to compost addition in September 2020 at each site.

In each plot, two 1 m2 quadrats for aboveground biomass clip plots were marked with railroad nails, 1 m from the edge of the narrow axis of the plot to avoid edge effects. One of the quadrats was surrounded by 4 ft tall fencing to create an exclosure within each plot. Two 5 m long transects for vegetation functional group composition were marked with railroad nails 1 m from the plot edges and 1 m from the clip plots. 

Responses

Bulk density was collected from 0-10 cm using a rigid metal cylinder (diameter X) hammered into the soil then excavated and placed in a plastic bag for transport to the lab to dry at 60C for 3d.

Aggregate stability was measured on 6 haphazardly-collected surface samples using methods from Herrick et al. YYYY. This method would not capture soils that could not be collected on the sieve (aka, category 0), but was consistent across all treatments. 

Infiltration rate was measured with a single ring infiltrometer (15.24 cm diameter) pounded into the soil in a randomly selected interspace (without perennial plants inside) to 10 cm depth. 444 mL of water was added and the time for all soil to infiltrate until soil surface was “just glistening” was recorded, up to 30 min. If the first 444 mL infiltrated within 30 min, a second 444 mL was added and the time for the water to infiltrate was recorded, up to 30 min. In 2022, we used a ruler to measured the time and depth over 3-4 time points and used a linear model to fit a regression line to then extract the time at which no water remains on the surface.

Vegetative cover and functional species richness and diversity was recorded along each transect and a pin flag was used to identify what dominant plants or ground cover intersected the transect every 5 cm in the following vertical categories: rooted or adhering to the soil surface (“ground level”), up to 50 cm (“herbaceous level”), 50 cm - 2 m (“shrub level”); no plants on the site were  >2 m. For analysis, we grouped plants into functional groups: annual vs. perennial, seedling (plants with only cotyledons up to the first two true leaves) vs. plant, and grass, forb, or woody. The litter category included fine and coarse herbaceous and woody debris and dung. Bare ground included soil and rock. 

Aboveground biomass was collected in the quadrats, both from the exclosure and in the open plot. We used a 1 m2 grid divided into 100 cm2 squares and randomly selected without replacement five squares to clip to ground level each year. Material was placed in paper bags, dried at 60C for 3d, and oxidized material was removed to capture material that was likely to have been alive in the previous 12 months. Material was weighed to 0.1 g.

Belowground biomass in the top 10 cm was collected in the same squares in the 1 m2 grid as the aboveground biomass with 0.9 cm diameter soil core. Soil was poured through a 2 mm sieve and roots were picked out from both the sieve and below (fine roots) with forceps. Roots were soaked and washed with water then dried for 60C for 3d and weighed to 0.XXX g.

Gravimetric water content was measured on soils aggregated from six 0.9cm diameter x 10 cm soil cores. Soils were sieved to 2mm and a subset of ~20g of field condition soil was placed in a tin, dried at 60C for 3d and re-weighed. 

Additionally, we partnered with Los Alamos National Labs to assess greenhouse gas fluxes in the compost and control plots using a Licor. 

Analyses were conducted in R using mixed effects models in the LMER package.