Root proliferation in native perennial grasses of arid Patagonia,Argentina

Pappophorum vaginatum is the most abundant C4 perennial grass desirable to livestock in rangelands of northeastern Patagonia, Argentina. We hypothesized that （1） defoliation reduce net primary productivity, and root length density and weight in the native species, and （2） root net primary productivity, and root length density and weight, are greater in P. vaginatum than in the other, less desirable, native species （i.e., Aristida spegazzinfi, A. subulata and Sporobolus cryptandrus）. Plants of all species were either exposed or not to a severe defoliation twice a year during two growing seasons. Root proliferation was measured using the cylinder method. Cylindrical, iron structures, wrapped up using nylon mesh, were buried diagonally from the periphery to the center on individual plants. These structures, initially filled with soil without any organic residue, were dug up from the soil on 25 April 2008, after two successive defoliations in mid-spring 2007. During the second growing season （2008-2009）, cylinders were destructively harvested on 4 April 2009, after one or two defoliations in mid- and/or late-spring, respectively. Roots grown into the cylinders were obtained after washing the soil manually. Defoliation during two successive years did reduce the study variables only after plants of all species were defoliated twice, which supported the first hypothesis. The greater root net primary productivity, root length den- sity and weight in P. vaginatum than in the other native species, in support of the second hypothesis, could help to explain its greater abundance in rangelands of Argentina.

Soil Organic Carbon Pools Under Switchgrass Grown as a Bioenergy Crop Compared to Other Conventional Crops

Switchgrass （Panicum virgaturn L.） has been proposed as a sustainable bioenergy crop because of its high yield potential, adap- tation to marginal sites, and tolerance to water and nutrient limitations. A better understanding of the potential effects of biomass energy crop production practices on soil biological properties and organic matter dynamics is critical to its production. Our objective was to evaluate changes in C pools under a warm-season perennial switchgrass in different soils compared to typically-grown crops col- lected at College Station, Dallas, and Stephenville, TX in February 2001. Sampling depths were 0-5, 5-15, and 15-30 cm. Switchgrass increased soil organic C （SOC）, soil microbial biomass C （SMBC）, mineralizable C, and particulate organic matter C （POM-C） com- pared to conventional cropping systems. Soil C concentrations were in the order： long-term coastal bermudagrass [Cynodon dactylon （L.） Pers.] 〉 switchgrass or kleingrass （Panicum coloratura L.） planted in 1992 〉 switchgrass 1997 〉 conventional cropping systems. Soil C concentrations tended to increase with increasing clay content. Greater microbial biomass C followed the order of Dallas 〉 College Station 〉 Stephenville, and ranged from approximately 180 mg C kg-1 soil at Stephenville to 1 900 mg C kg-1 soil at Dallas. Particulate organic C was more sensitive than other fractions to management, increasing as much as 6-fold under long-term coastal bermudagrass compared to conventional cropping systems. Our study indicated that conversion of conventional cropping systems into switchgrass production can sequestrate more SOC and improve soil biological properties in the southern USA.

Responses and sensitivity of N, P and mobile carbohydrates of dominant species to increased water, N and P availability in semi- arid grasslands in northern China

Aims We aimed to improve the understanding of the carbon and nutri-ent physiological responses and adaptation of semi-arid grassland plants to environmental changes.Methods We investigated plant leaf non-structural carbohydrate(NSC=solu-ble sugars+starch),nitrogen(N)and phosphorus(P)levels in an Inner Mongolian semi-arid grassland community treated with water,N and P additions for 8 years.Two dominant grasses(Agropyron cris-tatum(L.)Gaertn.,Stipa krylovii Roshev.)and two forbs(Artemisia frigida Willd.,Potentilla bifurca L.)were analyzed.Important Findings Water addition decreased plant leaf N and P concentrations,whereas N and P addition increased them,indicating that the semi-arid grassland studied suffers from a shortage of N and P sup-ply.Both N and P addition decreased the levels of soluble sugars,starch and thus also NSC in plant leaves,which may be attributed to(i)increased carbohydrate consumption associated with a higher growth rate,and(ii)a dilution effect of greater plant size under N and P addition.Water addition tended to increase the leaf NSC levels both in the grasses(+9.2%)and forbs(+0.6%only),which may be a result of increased photosynthesis of plants with increased water availability.Under conditions of ambient and increased water supply in the present study,N addition resulted in an N/P ratio of>16 in the grasses but a significantly lower N/P ratio of<11 in the forb species.This finding implies that growth of the two grass spe-cies will be limited mainly by P availability but the forbs will still be mainly limited by N supply if N deposition,alone or in combination with summer precipitation,continues to increase as predicted in Inner Mongolia.

Greater bud outgrowth of Bromus inermis than Pascopyrum smithii under multiple environmental conditions

Aims Tiller recruitment of perennial grasses in mixed-grass prairie primar-ily occurs from belowground buds.Environmental conditions,such as temperature,soil moisture and grazing can affect bud outgrowth of both invasive and native perennial grasses.Differential bud out-growth responses of native and invasive species to climate change and grazing could alter competitive interactions that have impli-cations for future land management.The aims of this work were to(i)compare how spring temperature altered bud outgrowth of native Pascopyrum smithii(Rydb.)Á.Löve(western wheatgrass)and introduced Bromus inermis Leyss.(smooth brome),(ii)compare how watering frequency altered bud outgrowth of these two species and(iii)evaluate how clipping interacts with spring temperature or watering frequency to affect P.smithii bud outgrowth.Methods Individual plants of B.inermis and P.smithii were harvested from North American mixed-grass prairie.Bud outgrowth from tillers of both species were evaluated under three spring temperature regimes(Average:12,18 and 24°C)and two watering frequen-cies(frequent and intermittent)in a growth chamber experiment.The response of P.smithii bud outgrowth to clipping was also examined.Important Findings Bromus inermis had more buds per tiller and initiated a greater pro-portion of these buds than P.smithii under all temperature and mois-ture conditions.Pascopyrum smithii bud development was reduced at 24°C.Intermittent watering did not significantly impact bud out-growth of either species.Clipping increased P.smithii bud mortality and reduced its bud development for the 2-week period of the study.The robust vegetative reproductive capacity of B.inermis under a range of environmental conditions is a key mechanism enabling the expansion of B.inermis into P.smithii-dominated mixed-grass prai-rie in North America.Mixed-grass prairie dominated by P.smithii experiencing repeated defoliation may require longer recovery times and be more susceptible to B.inermis invasion due to the negative impact of grazing on P.smithii bud outgrowth.Successful tiller recruitment and establishment of native perennial grasses via the bud bank will be necessary for mixed-grass prairie to be resilient to climate change,plant invasions and grazing.