In this final article on rangeland soil health, I want to focus on the nutrient cycle. How would you know if a pasture is showing signs of an efficient or good nutrient cycle? We monitor the nutrient cycle by looking for signs of living organisms (at both small and large scales) and how the litter builds up or decays (Figure 1).
Figure 1. Color coded monitoring indicators of nutrient cycling, water cycling, the biotic state, and energy flow that are involved in ecosystem processes.
(modified after Pyke et al. 2002; Pellant et al. 2005; Orchard 2013).
First, let’s look at the living organisms themselves or signs that they are around. We can evaluate such things as dung, from both livestock and wildlife. If the dung is decaying relatively quickly you know the nutrients are cycling (Figure 2). If the dung patties take several years to decay, the nutrients are likely going into the atmosphere rather than into the soil.
Higher order levels of life benefit from the cycling of nutrients. Beneficial insects such as dung beetles and pollinators provide valued services to the ecosystem. Diversity in plant life will promote higher level diversity in insect (Figure 3) and bird life.
Figure 2. Roller type dung beetle (upper right corner) and holes from other burying type dung beetles. Photo by A. Smart.
Figure 3. A bee feeding on dotted gayfeather. Photo by A. Smart.
The next thing to look at is litter. Litter is one of the best indicators because it is easy to monitor. If too much litter is present, it could inhibit tiller density and site productivity because it indirectly limits nitrogen mineralization. Sometimes, under these conditions, plants will show yellow or light green color in the leaves because nitrogen is not being mineralized quickly enough. Another sign to look for is low production of seed from perennial plants. Research has shown that removing litter by burning tallgrass prairie is equivalent to fertilizing with nitrogen in producing seed heads of warm-season grasses. If not enough litter is present (i.e. too much bare ground) biological activity will decrease because soil temperature may become too high and moisture too low from excessive evaporation.
I conducted a small experiment in Eastern South Dakota where I compared two areas (side-by-side) that had been grazed the previous year and an ungrazed control. The total aboveground biomass in the grazed area one year later was 5900 lbs/acre and 6100 lbs/acre in the ungrazed control. Interestingly, was the difference in litter and current year’s production between the two plots. The grazed area had a total of 3400 lbs/acre of live biomass and 2500 lbs/acre of litter. The ungrazed area had 1900 lbs/acre of live biomass and 4200 lbs/acre of litter. Below is a photo of the grazed and ungrazed areas. This is a good example of excess litter limiting both the energy flow and nutrient cycle.
Figure 4. Pasture that was grazed the previous year (on the left) and ungrazed (on the right). Notice the reddish color of the seed heads and darker green of the grass compared with the lack of seed heads and lighter color of grasses from the ungrazed area on the right. Photo by A. Smart.
Setting the correct stocking rate will usually resolve this problem in a few years if the pasture is in a season-long continuous grazing system and it is not too large to cause a grazing distribution problem. In this case, setting the stocking rate to achieve 50% utilization is the optimal target to maintain adequate residual herbage. If a pasture is in some type of rotation and the litter is too thick, then applying high stock density grazing for a short period in the spring will trample the litter, enhancing soil contact for rapid breakdown. Trampling during summer months can still be effective if precipitation is adequate. However, this usually is not the case as it may take winter snowfall to enhance soil contact for rapid breakdown the following spring. If a pasture is in a rotation and the litter is too low, then a seasonal deferment or a complete year of rest may be enough to provide adequate residual cover to build litter prior to the next growing season. General recommendations of minimum residual herbage levels for shortgrass, midgrass, and tallgrass rangeland should be 300 to 500, 750 to 1,000, and 1,200 to 1,500 lbs/acre respectively.
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