收费全文 | 1984篇 |
免费 | 136篇 |
国内免费 | 2篇 |
林业 | 166篇 |
农学 | 39篇 |
基础科学 | 15篇 |
365篇 | |
综合类 | 331篇 |
农作物 | 61篇 |
水产渔业 | 148篇 |
畜牧兽医 | 799篇 |
园艺 | 67篇 |
植物保护 | 131篇 |
2024年 | 4篇 |
2023年 | 18篇 |
2022年 | 28篇 |
2021年 | 54篇 |
2020年 | 56篇 |
2019年 | 79篇 |
2018年 | 67篇 |
2017年 | 59篇 |
2016年 | 62篇 |
2015年 | 48篇 |
2014年 | 64篇 |
2013年 | 88篇 |
2012年 | 151篇 |
2011年 | 180篇 |
2010年 | 86篇 |
2009年 | 60篇 |
2008年 | 141篇 |
2007年 | 113篇 |
2006年 | 145篇 |
2005年 | 117篇 |
2004年 | 102篇 |
2003年 | 113篇 |
2002年 | 93篇 |
2001年 | 11篇 |
2000年 | 19篇 |
1999年 | 12篇 |
1998年 | 15篇 |
1997年 | 13篇 |
1996年 | 4篇 |
1995年 | 14篇 |
1994年 | 9篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1990年 | 4篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1957年 | 2篇 |
1954年 | 2篇 |
Context
Resilience, the ability to recover from disturbance, has risen to the forefront of scientific policy, but is difficult to quantify, particularly in large, forested landscapes subject to disturbances, management, and climate change.Objectives
Our objective was to determine which spatial drivers will control landscape resilience over the next century, given a range of plausible climate projections across north-central Minnesota.Methods
Using a simulation modelling approach, we simulated wind disturbance in a 4.3 million ha forested landscape in north-central Minnesota for 100 years under historic climate and five climate change scenarios, combined with four management scenarios: business as usual (BAU), maximizing economic returns (‘EcoGoods’), maximizing carbon storage (‘EcoServices’), and climate change adaption (‘CCAdapt’). To estimate resilience, we examined sites where simulated windstorms removed >70% of the biomass and measured the difference in biomass and species composition after 50 years.Results
Climate change lowered resilience, though there was wide variation among climate change scenarios. Resilience was explained more by spatial variation in soils than climate. We found that BAU, EcoGoods and EcoServices harvest scenarios were very similar; CCAdapt was the only scenario that demonstrated consistently higher resilience under climate change. Although we expected spatial patterns of resilience to follow ownership patterns, it was contingent upon whether lands were actively managed.Conclusions
Our results demonstrate that resilience may be lower under climate change and that the effects of climate change could overwhelm current management practices. Only a substantial shift in simulated forest practices was successful in promoting resilience.Potassium has important physiological functions in eucalypt plantations, increasing their productivity when applied to soil via mineral fertilizers. There is interest in identifying alternative sources to KCl owing to its high cost and limited reserves. The aim of the study was to test the effect of replacing KCl with NaCl and phonolite rock powder. Two comparisons were made: (1) application of 283 kg ha?1 of KCl compared with that of 2125 kg ha?1 of phonolite rock powder (equivalent to 170 kg ha?1 of K2O in both treatments); (2) application of 139 kg ha?1 of NaCl compared with that of 183 kg ha?1 of KCl (equivalent to 2.33 kmol Na and K, respectively). Radial growth, soil water content, leaf water potential (Ψ), accumulated transpiration, stem volume and biomass increment, as well as water use efficiency (WUE) were evaluated. In the first comparison, both fertilizations presented equal values for all characteristics evaluated. In the second, the accumulated transpiration in trees fertilized with KCl was 17% higher than that in plants fertilized with NaCl. In contrast, the WUE was 20% higher in the trees fertilized with NaCl than in those fertilized with KCl, reflecting the lower water consumption for the same increment in stem volume and biomass. We conclude that phonolite rock powder and NaCl are possible substitutes for conventional K fertilization performed with KCl.
相似文献Nitrogen (N) is an important nutrient for re-vegetation during ecosystem restoration, but the effects of cover restoration on soil N transformations are not fully understood. This study was conducted to investigate N transformations in soils with different cover restoration ages in Eastern China.
Materials and methodsSoil samples were collected from four degraded and subsequently restored lands with restoration ages of 7, 17, 23, and 35 years along with an adjacent control of degraded land. A 15N tracing technique was used to quantify gross N transformation rates.
Results and discussionCompared with degraded land, soil organic carbon (SOC) and total N (TN) increased by 1.60–3.97 and 2.49–5.36 times in restoration land. Cover restoration increased ammonium and nitrate immobilization, and dissimilatory nitrate reduction to ammonium (DNRA) by 0.56–0.96, 0.34–2.10, and 0.79–3.45 times, respectively, indicating that restoration was beneficial for N retention. There were positive correlations between SOC content and ammonium and nitrate immobilization and DNRA, indicating that the increase in soil N retention capacity may be ascribed to increasing SOC concentrations. The stimulating effect of SOC on ammonium immobilization was greater than its effect on organic N mineralization, so while SOC and TN increased, inorganic N supply did not increase. Autotrophic and heterotrophic nitrification increased with increasing SOC and TN concentrations. Notably, heterotrophic nitrification was an important source of NO3??N production, accounting for 47–67% of NO3??N production among all restoration ages.
ConclusionsThe capacity of N retention was improved by cover restoration, leading to an increase in soil organic carbon and total N over time, but inorganic N supply capacity did not change with cover restoration age.
相似文献