Field evidences for the positive effects of aerosols on tree growth.

Theoretical and eddy covariance studies demonstrate that aerosol-loading stimulates canopy photosynthesis, but field evidence for the aerosol effect on tree growth is limited. Here, we measured in situ daily stem growth rates of aspen trees under a wide range of aerosol-loading in China. The results showed that daily stem growth rates were positively correlated with aerosol-loading, even at exceptionally high aerosol levels. Using structural equation modeling analysis, we showed that variations in stem growth rates can be largely attributed to two environmental variables covarying with aerosol loading: diffuse fraction of radiation and vapor pressure deficit (VPD). Furthermore, we found that these two factors influence stem growth by influencing photosynthesis from different parts of canopy. Using field observations and a mechanistic photosynthesis model, we demonstrate that photosynthetic rates of both sun and shade leaves increased under high aerosol-loading conditions but for different reasons. For sun leaves, the photosynthetic increase was primarily attributed to the concurrent lower VPD; for shade leaves, the positive aerosol effect was tightly connected with increased diffuse light. Overall, our study provides the first field evidence of increased tree growth under high aerosol loading. We highlight the importance of understanding biophysical mechanisms of aerosol-meteorology interactions, and incorporating the different pathways of aerosol effects into earth system models to improve the prediction of large-scale aerosol impacts, and the associated vegetation-mediated climate feedbacks.

Increased phosphate uptake but not resorption alleviates phosphorus deficiency induced by nitrogen deposition in temperate Larix principis-rupprechtii plantations.

The imbalance between nitrogen (N) and phosphorus (P) deposition may shift temperate ecosystems from N- to P-limitation. However, it is unclear how the imbalanced N : P input affects the strategies of plants to acquire P and, therefore, the growth of plants and the competition among species. We conducted a 4-yr N-addition experiment in young and mature larch (Larix principis-rupprechtii) stands. Plant growth and P acquisition strategies were assessed for larch and understorey vegetation. N addition stimulated the aboveground productivity of understorey vegetation in the young stand and larch in the mature stand, with other species unaffected. The competitive advantages of understorey vegetation in the young stand and larch in the mature stand were associated with their high stoichiometric homoeostasis. To maintain the N : P homoeostasis of these species, an increase in phosphatase activity but not P resorption efficiency increased the supply of P. Additionally, N addition accelerated P mineralization by decreasing the fungal-to-bacterial ratios and improved uptake of soil P by increasing the arbuscular mycorrhizas-to-ectomycorrhizas ratios. Our results suggest that plants with high stoichiometric homoeostasis could better cope with N deposition-induced P-deficiency. Although P resorption efficiency showed little plasticity in response, plants activated a variety of P-acquisition pathways to alleviate the P-deficiency caused by N deposition.

How inhibiting nitrification affects nitrogen cycle and reduces environmental impacts of anthropogenic nitrogen input.

Anthropogenic activities, and in particular the use of synthetic nitrogen (N) fertilizer, have doubled global annual reactive N inputs in the past 50-100 years, causing deleterious effects on the environment through increased N leaching and nitrous oxide (N2 O) and ammonia (NH3 ) emissions. Leaching and gaseous losses of N are greatly controlled by the net rate of microbial nitrification. Extensive experiments have been conducted to develop ways to inhibit this process through use of nitrification inhibitors (NI) in combination with fertilizers. Yet, no study has comprehensively assessed how inhibiting nitrification affects both hydrologic and gaseous losses of N and plant nitrogen use efficiency. We synthesized the results of 62 NI field studies and evaluated how NI application altered N cycle and ecosystem services in N-enriched systems. Our results showed that inhibiting nitrification by NI application increased NH3 emission (mean: 20%, 95% confidential interval: 33-67%), but reduced dissolved inorganic N leaching (-48%, -56% to -38%), N2 O emission (-44%, -48% to -39%) and NO emission (-24%, -38% to -8%). This amounted to a net reduction of 16.5% in the total N release to the environment. Inhibiting nitrification also increased plant N recovery (58%, 34-93%) and productivity of grain (9%, 6-13%), straw (15%, 12-18%), vegetable (5%, 0-10%) and pasture hay (14%, 8-20%). The cost and benefit analysis showed that the economic benefit of reducing N's environmental impacts offsets the cost of NI application. Applying NI along with N fertilizer could bring additional revenues of $163 ha(-1)  yr(-1) for a maize farm, equivalent to 8.95% increase in revenues. Our findings showed that NIs could create a win-win scenario that reduces the negative impact of N leaching and greenhouse gas production, while increases the agricultural output. However, NI's potential negative impacts, such as increase in NH3 emission and the risk of NI contamination, should be fully considered before large-scale application.