Phosphorus (P) availability in tropical soils is often limited due to strong adsorption onto iron (Fe) and aluminum (Al) oxides. Silicon (Si) has been reported to increase P availability, often attributed to competition between Si and P for adsorption sites. However, the mechanisms underlying this interaction remain unclear because most studies have used silicate sources that increase soil pH. This study evaluated the role of Si in the temporal redistribution of soil P fractions in tropical soils. Two contrasting tropical soils (clayey and sandy clay loam) were incubated with four P levels (0, 50, 100, and 200 mg kg−1) and four Si levels (0, 240, 480, and 960 mg kg−1) using hydrophilic amorphous silica as the Si source, which does not increase soil pH. Soil samples were collected after 15, 30, 90, 180, 270, and 360 days of incubation to determine labile, moderately labile, and non-labile P fractions, as well as soil Si and pH. Phosphate fertilization increased moderately labile inorganic P associated with Fe and Al oxides. Silicon temporarily increased labile P and reduced occluded P during the early incubation period, whereas its effects on P fractions varied with soil texture and incubation time. Overall, Si influenced the redistribution of both inorganic and organic P pools, indicating that its effects on P dynamics in tropical soils are transient and strongly dependent on soil texture and incubation time rather than representing a persistent increase in P availability.
Soil pore structure directly modulates the intensity and pathway of nitrogen (N) migration and environmental loss within the soil profile, thus serving as a critical bridge that links pore-scale processes to cropland ammonia (NH3) volatilization. However, the effect of soil pore structure on NH3 emissions has long been implicitly embedded in apparent indicators, such as fertilization depth and bulk density, in previous observational and modelling studies. In this study, we investigated contrasting pore structures resulting from four maize fertilization practices: surface urea application (BC treatment) vs. surface urea application with surface soil crusting (about 2 cm; CM treatment); subsurface urea application (10 cm; DP treatment) vs. subsurface urea application (10 cm) with soil compaction (0–20 cm; CP treatment). We simultaneously measured soil pore structure, relative gas diffusivity, nitrogen migration in the soil profile, and soil NH3 volatilization fluxes. Experimental results reveal that soil pore structure plays a dual role in regulating soil NH3 volatilization, by regulating substrate concentration (i.e., ammonium, NH4+), and governing the profile distribution of NH4+ and NH3. Under deep placement conditions, anoxic conditions suppressed nitrification in CP, resulting in higher NH4+ concentration around the fertilizer hotspot than in DP. However, compaction restructured the soil pore architecture through compression, decreasing connectivity volumes and relative gas diffusivity by 133.94 % (from −128808 to 43716) and 82.13 %, which ultimately reduced the NH3 volatilization flux in CP by 37.38 % compared with DP. Although CM formed a distinct dense layer in the topsoil (2.31 cm) and reduced the connectivity volumes and relative gas diffusivity by 49.34 % and 32.34 % compared with BC, it unexpectedly increased NH3 emissions by 17.69 %. This is because the crust-induced soil warming (+3.43 % increase) and the consequent increases in urease activity (+114.36 %) and NH4+ concentration (+14.90 %) overshadowed the pore’s blocking effect. After isolating the pore effect using an NH3 compensation point model, results showed that the crusting-induced reduction in porosity decreased NH3 volatilization flux by 10.23 %. This study elucidates the dual role of soil pore structure in regulating NH3 volatilization flux and further reveals that the trade‑off between these two roles determines the net intensity of NH3 volatilization.
Under intensive short-rotation forestry, nitrogen (N) concentrations and potential transformation rates can differ disproportionately, but their aggregate-scale relationships remain unclear. We compared first- to third-rotation Eucalyptus plantations (G1–G3) in subtropical China, with five independent six-year-old stands per rotation. Organic and inorganic N fractions and their net changes were measured in 30-day incubations of separated aggregates and matched bulk soils from 0 to 20 and 20 to 40 cm. Relative to G1, G3 had 28% and 51% lower total N concentrations, but 81.3% and 72.4% lower measured bulk-soil potential net mineralization in topsoil and subsoil, respectively. Smaller fractions generally had higher inorganic N and acid-hydrolyzable ammonium, amino sugar and amino acid N concentrations, whereas >2 mm aggregates had the highest mean net mineralization and nitrification rates. Descriptive comparisons of group means showed the largest proportional G3–G1 declines in net mineralization and nitrification rates in the >2 mm fraction. Their mean amino acid N depletion rates were 88% and 80% lower, compared with 44% and 67% lower concentrations in topsoil and subsoil, respectively. Of the mass-weighted totals from separated fractions, >2 mm aggregates accounted for 71% and 73% of net mineralization in G1 topsoil and subsoil, respectively, versus 57% and 61% in G3. Mean reconstructed mineralization rates exceeded matched bulk-soil measurements in all groups. These findings show differences in aggregate-scale functional partitioning across rotation histories, with a smaller but persistent majority share of the >2 mm fraction in reconstructed potential net N mineralization.
Declining soil health threatens food security and the livelihoods of millions of smallholder farmers across the Global South, while increasing their vulnerability to climate change. Yet, existing soil health frameworks are mostly focused on the Global North and fail to guide practical decision-making that is tailored to local contexts. Here, we conceptualize soil health as a foundational pillar of agronomic performance of smallholder crop systems and present a purpose-led and adaptive soil health framework to bridge soil health gaps. Similar to the yield gap concept, we define the ‘soil health gap’ as a benchmark of the soil’s current performance relative to its potential under improved management with respect to ecological functions, including crop production, carbon storage, nutrient cycling and water regulation. Integration of soil chemical, physical, and biological indicators provides a tool to identify the specific soil health gap to be addressed in a given context, and to monitor improvement. The framework follows four iterative phases: (i) Contextualize: characterize local conditions and engage stakeholders; (ii) Diagnose: identify functional soil health constraints using context-sensitive indicators; (iii) Advise: translate diagnostics into actionable recommendations; and (iv) Monitor: track changes in soil health, agronomic and livelihood outcomes. Based on examples from Peru, Kenya and Vietnam we illustrate the applicability of the framework and draw lessons from existing experiences in contrasting smallholder contexts. Finally, we highlight priorities for further research and implementation. The proposed approach bridges scientific knowledge and smallholder realities, proposing a pathway to address soil health gaps and enhance the resilience of tropical smallholder crop systems.
Widespread soil organic carbon (SOC) loss under accelerating anthropogenic activities threatens both soil biodiversity and carbon persistence. Carbon decline is often accompanied by soil structure disruption, reorganizing the microhabitats for microbe-carbon interactions toward smaller aggregate domains. A key question is how such structural reorganization affects microbial regulation of soil carbon turnover. Here, we used twelve Mollisol croplands along an agricultural chronosequence of SOC decline to trace microbial diversity and carbon decomposition in relation to changes in aggregate structure. As SOC declined, aggregate-size distribution shifted progressively toward a greater contribution of microaggregates, which consistently harbored lower microbial diversity than macroaggregates. Aggregate-specific diversity responses differed markedly between aggregate fractions: macroaggregates showed persistent diversity loss, whereas microaggregates exhibited delayed but accelerated loss under stronger SOC depletion. These contrasting trajectories underlay the nonlinear decline in bulk-soil diversity, with limited change at higher SOC levels but sharp losses below a threshold of 27‒29 g C kg−1. Despite declines in microbial diversity and SOC stocks, mineralization of both added 13C-labelled glucose and vanillin increased in bulk soils. Notably, this acceleration occurred primarily within microaggregates, where genes associated with oxidative carbon degradation and respiratory metabolism were enriched under low-SOC conditions, while substrate mineralization within macroaggregates remained relatively insensitive to SOC decline. Overall, our findings reveal that aggregate fragmentation drives SOC-depleted soils toward a low microbial diversity yet high carbon turnover state, primarily by increasing the importance of microaggregate domains with enhanced oxidative metabolism. This shift may increase the vulnerability of remaining soil carbon to further loss. Incorporating microscale soil structure and associated biological processes is therefore critical for predicting soil carbon persistence and designing management strategies to conserve biodiversity and rebuild SOC stocks.
