Long-Term C and N Changes in Soil and Mixed Native Grass Canopies Under Different Grassland Management Strategies.

This study was conducted to determine whether soil and grass C and N could be manipulated through canopy management and different grass mixtures.  Effects of forced management (burning in early spring, mowing and residue removal at grass anthesis, or no management) on grass (native grass plantings of mixtures of cool season species, warm season species, or combined cool and warm season species) growth as well as C and N levels in soil and grass tissues were investigated at Brookings, SD, on a Barnes clay loam (fine-loamy, superactive, frigid Calcic Hapludoll).  Soil C (0-15 cm depth) increased linearly over the 8 years of the experiment.  Soil C accumulation rate was significantly greater under no management (714 kg C /ha/year) than burn treatment (333 kg C /ha/year) while that under mow was intermediate (504 kg C /ha/year).  In years 7 and 8, soil C (burn 22.0 g/kg; no management 23.4; mow 23.2; P=0.009) and soil C/N ratio (burn 11.4; no management 11.9; mow 11.8; P=0.02) were all reduced by burn treatments.  The burn treatment significantly reduced grass biomass (burn 2730 kg/ha; no management 4656; mow 3421; P=0.0001).  Cool season grasses also produced less biomass than the other grass mixtures (cool season 2814 kg/ha; warm season 3989; and mix 4004; P=0.0001).  Significant management by grass mixture (2-way) interactions for biomass (P=0.02) resulted from increases in biomass for the warm and warm/cool grass mixtures under the no management treatment.  Grass C/N ratio was less under the mow treatment (burn 48.9; no management 51.4; mow 43.8; P=0.0001).  In conclusion, the burn treatment was detrimental to soil C concentration and C/N ratio as well as to the growth of cool season grasses.  Mow and remove management, which would be compatible with cellulosic biomass production, showed values of soil C concentration and C/N ratios comparable with those seen under the no management treatments.  However, mow and removed also reduced grass biomass as well as grass tissue C/N ratio.  Additional time-course data is being collect to determine if mow and remove would be sustainable in terms of long-term soil resource quality.