How Much Soil Microbial Biomass does a Healthy Grassland Need
Llewellyn L. Manske PhD
Scientist of Rangeland Research
Dickinson Research Extension Center
701-456-1118
llewellyn.manske@ndsu.edu
While working at Sidney ARS Station, MT, Wight and Black (1972, 1979) concluded that available mineral nitrogen was the major grass growth limiting factor in Northern Plains native grasslands and they determined that in order to produce herbage biomass at biological potential levels, 100 lbs/ac or greater of available mineral nitrogen was required. Grasslands with long-term traditional practices had a mean of 59 lbs N/ac available. Grasslands that had less than 100 lbs N/ac produced herbage biomass an average of 49.6% below the weight of herbage produced per inch of precipitation received on grasslands that had 100 lbs N/ac or greater. This data showed that water use efficiency of grass plants could be greatly improved with increases in available mineral nitrogen at quantities of 100 lbs/ac or greater.
Manske (1989, 1992) quantified the levels of available mineral nitrogen on grasslands with a variety of management practices: long-term nongrazed with control of no burns at 31.2 lbs N/ac or with 1 to 4 burns at 26.3 lbs N/ac; standard nongrazed at 39.5 lbs N/ac; deferred-rotation at 31.2 lbs N/ac; 6.0 m seasonlong at 61.6 lbs N/ac; 4.5 m seasonlong at 76.7 lbs N/ac; and twice-over rotation at 177.8 lbs N/ac. This data showed that the type of management affects the quantity of available mineral nitrogen, and that any type of grazing management, except the deferred-rotation, provided greater available mineral nitrogen than nongrazed or burned grasslands, and that the twice-over rotation was the only known practice that provided greater than 100 lbs N/ac.
Manske (2010) determined that full activation of the four major internal grass growth mechanisms would not occur unless 100 pounds of mineral nitrogen or greater were available.
Most intact Northern Plains grasslands contain 5 to 6 tons/ac of organic nitrogen and do not require application of supplemental nitrogen. Organic nitrogen is not available for plant growth and must be transformed by soil microbes into mineral nitrogen. Northern Plains grasslands are not deficient in nitrogen but are deficient of soil microbe biomass because of the types of traditional management used.
Coleman et al. (1983) described the soil rhizosphere microorganisms and the basic science of the ecosystem biogeochemical processes that soil microbes performed. The quantity of biogeochemical processes conducted in the soil of a grassland ecosystem is dependent on the quantity of the rhizosphere microbe biomass, which is limited by access to simple carbohydrate energy (Curl and Truelove 1986). The microflora trophic level (bacteria and fungi), which compose the greatest biomass of the rhizosphere, lack chlorophyll and cannot capture their own carbon energy.
Grassland plants only have small quantities of natural leakage of carbon base materials from their roots. This low amount of carbon compound leakage is typically barely enough to sustain a small rhizosphere biomass of around 55 kg/m3 (245 tons/ac) that can transform only around 25 lbs N/ac.
Grass lead tillers capture and fix greater quantities of carbon energy than needed for normal growth and maintenance during the vegetative growth stages (Coyne et al. 1995). This relatively large amount of surplus carbon energy can be exudated from the grass tillers through the roots into the rhizosphere by partial defoliation by grazing (Holland et al. 1996) which would increase the rhizosphere microbe biomass and biogeochemical activity resulting in greater quantities of organic nitrogen transformed into mineral nitrogen.
Gorder, Manske, and Stroh (2004) documented that 15 days of cattle grazing during lead tiller vegetative growth between the 3.5 new leaf stage and the flower stage (1 June to 15 July) caused the rhizosphere volume to more than double in size (from 3541 cm3/m3 to 7183 cm3/m3) during the growing season with an increase of 463% greater than that on the seasonlong grazing treatment (at 1552 cm3/m3).
Manske (2012) showed that severely degraded grasslands that had reached a low rhizosphere biomass of 52.23 kg/m3 (233.04 tons/ac) and was transforming only 24.18 lbs N/ac could be improved after 6 years of twice-over rotation grazing at near 100% of seasonlong stocking rates by building the rhizosphere biomass to a minimum effective level of 214.34 kg/m3 (956.35 tons/ac) and transforming 99.23 lbs N/ac (just short of 100 lbs/ac) for an increase of 410%.
Rhizosphere biomass data is simpler to work with in metric figures related to a small amount of land of one cubic meter. However, most people do not visualize in the metric system. To convert metric rhizosphere biomass to commonly understood values, multiply kg/m3 by 4.462 to get tons/ac on a large amount of land (an acre) that is 39 inches deep, thus 214.34 kg/m3 equals 956.39 tons/ac of rhizosphere biomass which is the lowest rhizosphere biomass that can transform 100 pounds of mineral nitrogen per acre.
The greatest rhizosphere biomass that has been determined so far on grasslands managed by the twice-over system was 406.44 kg/m3 that converts to 1,813.46 tons/ac or 3,626, 922.21 lbs/ac and would transform around 188.17 lbs N/ac. The collective data accumulated to date, strongly suggest that it requires around 2.16 kg/m3 or 9.64 tons of rhizosphere biomass per acre to transform one pound of mineral nitrogen per acre. A healthy grassland requires that a minimum of 100 pounds of mineral nitrogen or greater be transformed from the organic nitrogen, which requires that the grazing management practice implemented must be able to maintain a viable rhizosphere biomass of slightly greater than 216 kg/m3 or 964 tons/ac each year.
Annual maintenance of rhizosphere microbes at a biomass of at least 216 kg/m3 and 964 tons/ac requires that the surplus carbon energy captured by native grass lead tillers during vegetative growth stages between 1 June and 15 July (45 day period) needs to be exudated to the rhizosphere microbes by partial defoliation by grazing for 7 to 17 days (never less or more) that removes 25% to 33% of the aboveground herbage on 60% to 80% of the lead tiller population of cool and warm season grasses on each of three to six pastures of native grassland managed by the twice-over rotation strategy (Manske 2018).
The larger the rhizosphere microbe biomass, the greater the quantity of available mineral nitrogen, and the greater the quantity of grass herbage produced. The upper limit of the achievable quantity of available mineral nitrogen on Northern Plains grasslands has not been determined yet, but it is expected to be around 200 pounds of mineral nitrogen per acre which would require about 2000 tons of rhizosphere biomass per acre or about 4 million pounds of rhizosphere microbes on an acre that was 39 inches deep.
The cited literature can be found on the web site below.
For More Information Go To:
https://www.ndsu.edu/agriculture/ag-hub/publications/grazing-handbook
