Effects of western juniper(Juniperus occidentalis)control on ecosystem nitrogen stocks in central Oregon,USA

Aims In the Oregon of USA,the control of western juniper(Juniperus occidentalis)is an accepted rangeland management practice to restore sagebrush steppe habitats of importance to wildlife and livestock.The effects of juniper cutting on ecosystem nitrogen,however,have not been well addressed although woody plant control has important implications for local watershed management and regional nitrogen pools.Methods We quantified ecosystem nitrogen stocks in two adjacent watersheds,comprised of a treated watershed(most juniper removed)and an untreated watershed(juniper not removed).Thirteen years after juniper removal,we measured aboveground nitrogen stocks for juniper trees,shrubs,grasses and litter in both watersheds.We also measured belowground nitrogen stocks(roots and soil)in both watersheds at two soil depths(0–25 and 25–50 cm).Important Findings Aboveground nitrogen stocks were 6.9 times greater in the untreated than in the treated watershed considering the much larger aboveground biomass.However,root nitrogen stocks were 3.1 times greater in the treated one due to the gain of understory root biomass associated with juniper cutting.Soil nitrogen stocks at both 0–25 and 25–50 cm depths were not affected by juniper removal.Overall,total ecosystem nitrogen stocks did not differ between the treated(9536 kg N ha−1)and untreated(9456 kg N ha−1)watersheds.The greatest ecosystem nitrogen accumulation(at least 95%total ecosystem nitrogen)resided belowground(soil 0–50 cm and roots)in both watersheds.This study provides evidence that the benefits of juniper removal can be attained without significantly affecting the capacity of ecosystem nitrogen storage.

Dynamics of organic carbon and nitrogen in deep soil profile and crop yields under long-term fertilization in wheat-maize cropping system

Soil organic carbon(SOC)and nitrogen(N)are two of the most important indicators for agricultural productivity.The primary objective of this study was to investigate the changes in SOC and N in the deep soil profile(up to 100 cm)and their relationships with crop productivity under the influence of long-term(since 1990)fertilization in the wheat-maize cropping system.Treatments included CK(control),NP(inorganic N and phosphorus(P)fertilizers),NPK(inorganic N,P and potassium fertilizers),NPKM(NPK plus manure),and M(manure).Crop yield and the properties of topsoil were measured yearly from 2001 to 2009.C and N contents were measured at five different depths in 2001 and 2009.The results showed that wheat and maize yields decreased between 2001 and 2009 under the inorganic fertilizer(NP and NPK)treatments.The average yield between 2001 and 2009 under the NP,NPK,NPKM,and M treatments(compared with the CK treatment)increased by 38,115,383,and 381%,respectively,for wheat and 348,891,2738,and 1845%,respectively,for maize.Different long-term fertilization treatments significantly changed coarse free particulate(cf POC),fine free particulate(ff POC),intramicroaggregate particulate(i POC),and mineral-associated(m SOC)organic carbon fractions.In the experimental years of 2001 and 2009,soil fractions occurred in the following order for all treatments:m SOC>cf POC>i POC>ff POC.All fractions were higher under the manure application treatments than under the inorganic fertilization treatments.Compared to the inorganic fertilization treatments,manure input enhanced the stocks of SOC and total N in the surface layer(0–20 cm)but decreased SOC and N in the deep soil layer(80–100 cm).This reveals the efficiency of manure in increasing yield productivity and decreasing risk of vertical loss of nutrients,especially N,compared to inorganic fertilization treatments.The findings provide opportunities for understanding deep soil C and N dynamics,which could help mitigate climate change impact on agricultural production and maintain soil health.