Tradeoffs between forage quality and soil fertility: Lessons from Himalayan rangelands

Abstract

A B S T R A C TPastoralists on Eurasian rangeland often believe that traditional management practices involving moderate rotational grazing ensure sustainable yield, increase plant digestibility and promote biodiversity. To assess the generality of these suppositions we compared biomass production, community leaf C, N and P stoichiometry, leaf trait diversity and carbon and nutrient pools in root and soil for moderately grazed vs. ungrazed plots in Tibetan alpine meadows. We used five leaf traits (leaf C, leaf N and leaf P concentrations; SLA: specific leaf area, and LDMC: leaf dry matter content) as indicators of plant digestibility and rangeland quality. We measured these foliar traits and the ramet numbers for component species in moderately grazed plots as well as in exclosures (3-11 years) at five sites across the Qinghai-Tibetan plateau. Community weighted mean (CWM) trait values and functional dispersion (FDis) were used to quantify the mean and the variance in the distribution of trait values, respectively. Both the leaf P CWM and leaf P FDis generally increased under grazing and the LDMC CWM decreased, leading to improved plant digestibility and rangeland quality (e.g. high, community-wide leaf nutrients). The leaf C CWM , leaf N CWM and SLA CWM increased under grazing but the FDis of these traits tended to decrease. Grazing generally increased species diversity but decreased aboveground biomass, organic carbon, and nutrient concentrations in soil and root, especially decreasing root nitrogen and soil available phosphorus. Both root biomass and the leaf C:N CWM decreased in grazed plots at wet sites, but increased at dry sites. The community-wide increase and greater interspecific diversity in leaf nutrient concentrations coupled with decreasing LDMC CWM show that grazing induced an increase in plant digestibility, nutrient concentrations, and nutrient diversity in these alpine meadows. However, this increase in forage quality comes at the cost of losses in both carbon stock and nutrient availability that depress biomass production. Our findings and a review of related literature suggest that traditional grazing practices involve a trade-off between short-term yield and sustainability, a management challenge that must be addressed on rangelands with low soil fertility.

Traditional grazing regimes promote biodiversity and increase nectar production in Tibetan alpine meadows

“…ungrazed, lightly grazed, moderately grazed, and over grazed). In 2014, most of the livestock were move to pastures in higher elevations from late May to September (Niu et al, 2016). At that time, five 1 m × 1 m plots were randomly established in each of four pastures to measure nectar production, plant traits, and community composition (details regarding the location of the sites and plots are provided in Supplemental Materials Fig.…”

“…warming) and in land use practices (e.g. nitrogen deposition) over the past three decades have jeopardized the sustainability of these rangelands (Harris, 2010;Niu et al, 2016) both in terms of natural services (e.g., nectar production for indigenous insect pollinators) and agricultural services (e.g., the production of grazing crops for agriculture) (Bennett et al, 2009;Niu et al, 2016). The linkages between natural and agricultural services are complex and at times problematic.…”

“…For example, based on grazing practices over hundreds of years, traditional Tibetan pastoralists believe that light to moderate grazing ensures sustainable forage yields and actually promotes the otherwise diverse natural services grasslands provide (e.g. biodiversity and nectar production) (Yeh, 2005;Niu et al, 2016) .The production of nectar is crucial not just to native insect pollinators but also to apiculture, which has become a key industry in the eastern Tibetan rangeland over the last three decades (Mu et al, 2014). In contrast, there is some evidence that overgrazed (or ungrazed) rangelands yield a greater nectar production in large part because both land use practices favor the growth of nectariferous forbs species albeit with a detrimental effect on biodiversity (Xie et al, 2008).…”

Association of soil bacterial diversity with plant community functional attributes in alpine meadows

“…如土壤氮和磷通常是草地生产力的限制因子, 在本研 究的高寒草甸自然生态系统中土壤磷资源对高寒草地 初级生产力限制作用更为显著 [44] ; 土壤氮和磷共同限 制生产力的作用导致本研究中土壤细菌多样性随土壤 全氮的增加而增加, 并且与土壤磷资源呈显著正相 关 [5] . 研究结果也表明, 土壤细菌多样性与土壤速效氮 显著负相关, 可能是由于地下生物量随土壤速效氮增 加而减小 [45] , 从而降低土壤细菌多样性 [46] ; 而土壤细 菌多样性的增加, 会反馈影响到土壤速效氮的降低, 如通过加速土壤细菌的脱氮作用 [47] .…”

Pika burrow and zokor mound density and their relationship with grazing management and sheep production in alpine meadow

“…, Niu et al. ). At present, there are about fifteen million yaks and fifty million Tibetan sheep which, together with small mammals (pikas and zokors), are well adapted to the hypoxic environment (Zhao , Sun et al.…”