Nutrient enrichment driven by canopy rainfall redistribution: Mechanism, quantification, and pattern

Vegetation canopies intercept and redistribute rainfall into throughfall and stemflow, which transfer substantial amounts of elements into the soil, influencing soil microbial community, plant survival, and plant community succession. Despite advancements in ecohydrological research, the implication of nutrient enrichment resulting from this redistribution of rainfall by canopies remains largely unexplored. To address this gap, we conducted a systematic review of 1020 papers published between 2000 and 2022, gathering data on nutrient concentration and enrichment for critical ions (including K+, Na+, Ca2+, Mg2+, NH 4 + , Cl−, NO 3 − and SO 4 2− ) from the Web of Science and Chinese Knowledge Infrastructure databases. We aimed to synthesize the mechanisms, quantify the enrichments, and identify global patterns of nutrient enrichment in stemflow and throughfall across climate zones, and vegetation types and ecosystems. The results of this study indicate that stemflow exhibits, on average, 2.1 times greater ion concentration (6.13 mg L−1) compared to throughfall. In particular, among the investigated ions, SO 4 2− (12.45 and 6.32 mg L−1) for stemflow and throughfall, respectively, and Cl− (9.21 and 4.81 mg L−1) exhibit the highest concentrations in both rainfall redistribution components, while K+ (13.7 and 5.8) and Mg2+ (5.6 and 2.8) have the highest enrichment factors. Across climate zones, throughfall and stemflow show the lowest ion concentrations but the highest enrichment factors in extremely humid regions. Along the temperature gradient, ion concentrations are the highest in cold climates with no clear patterns observed for enrichment factors with increasing temperature. In addition, shrubs, conifers, mixed forests, and artificial ecosystems demonstrate enrichment factors 1.1 to 3.0 times greater than those of trees, broad- leaved plants, pure forests, and natural ecosystems. These findings emphasize the need for increased attentions to artificial ecosystems, such as urban and agricultural ecosystems, which often received limited research focus, especially regarding shrubs and conifers exhibiting stronger nutrients enrichment capabilities. Future investigations should integrate soil moisture analysis to better understand the impact of rainfall redistribution on the nutrient enrichment processes, patterns, and nutrient balance in global terrestrial ecosystems.