Soil nanocolloids and associated phosphorus in natural and managed ecosystems

  • Bodennanokolloide und assoziierter Phosphor in natürlichen und bewirtschafteten Ökosystemen

Zhang, Qian; Klumpp, Erwin (Thesis advisor); Schäffer, Andreas (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022


Natural colloidal particles (diameter < 1,000 nm) are the smallest particulate phase in soils. They are rich in soil organic carbon (SOC), iron-/aluminium- (hydr)oxides and clay minerals and play major roles in determining e.g. ion exchange capacity and in regulating key biogeochemical processes. Natural colloids, including nanoparticles or nanocolloids (1-100 nm) have a large specific surface area and are rich in surface charge, and thus, they can immobilize nutrients and are a main factor affecting the availability of phosphorus (P) in soils and rivers. Furthermore, P carried by colloids can enhance the transport of soil P to external water bodies. Consequently, it is important to understand how soil colloids and their functions vary in diverse terrestrial ecological systems (i.e. forest, savanna and arable soils) and for different anthropogenic disturbances (i.e. livestock grazing and fertilization application). Amazonian Dark Earths (or Terra Preta de Indico) are known as highly fertile soils that can maintain elevated crop yields for centuries. While this fertility was frequently ascribed to the presence of black carbon, the availability and colloidal binding of major nutrients received limited attention. The size distribution and the elemental compositions of water-dispersible colloids (WDC) in both forested and cultivated Terra Preta topsoils (0-10 cm, Anthrosols), as well as in their adjacent non-Terra Preta controls (Acrisols) via Asymmetrical Flow Field Flow Fractionation (AF4) showed that WDC in Terra Preta soils contained a significant proportion of organo-mineral associations in the size range 30-300 nm, whereas, in contrast, water-dispersible nanoparticles with a diameter <30nm were dominant in the adjacent Acrisols. The shifts to larger WDC sizes in the Terra Preta soils went along with elevated pH values, as well as with elevated contents of Si, Al, Fe, Ca and organic matter-containing particles. Also P concentrations were enriched in both the water-extractable phase (WEP) and WDC extracts of Terra Preta soils relative to the adjacent Acrisols. It was assumed that the higher pH values and Ca ion concentrations promoted the involvement of soil organic matter (SOM) into the formation of larger-sized colloids consisting of kaolinite-like clay minerals, iron oxides and Ca ions in the Terra Preta soils. The elevated content of Ca in Terra Preta soil colloids may also contribute to the retention of P, likely via bridging of anionic P like orthophosphate to SOM. Preventing soil acidification is thus not only to be recommended for Acrisols, but also for maintaining colloidal structures and related fertility in Terra Preta soils. In grasslands, savannas, and other dryland ecosystems across the globe, woody plants are encroaching due to livestock grazing and fire suppression. These major land cover changes influence soil colloidal properties, with implications for soil C, N, and P cycles. Results showed that surface soils (0 - 10 cm) beneath oak and juniper canopies had smaller WDC sizes and elevated colloidal organic carbon (OC) and P concentrations, especially in nanocolloid (< 30 nm) and fine colloid (30 - 240 nm) size fractions. Woody encroachment enriched Ca, Fe, Al, Si and Mg in WDC in the undisturbed area, but did not alter their concentrations in grazed or burned areas. Colloidal P, mainly occurring as orthophosphate and orthophosphate diesters as revealed by liquid-state 31P-nuclear magnetic resonance spectroscopy (31P-NMR), was associated with colloidal OC-Ca complexes instead of clay minerals or Al- and Fe (hydro)oxides. The findings of this thesis suggested that woody encroachment may strengthen the retention of OC and P by natural colloids, consequently increasing C and P pools in savanna soils. Nano and colloidal particles play important roles in P migration and loss from agricultural soils; however, little is known about their relative distribution in arable crop soils under varying agricultural geo-landscapes at the regional scale. The AF4 results showed that WDCs from surface soils (0‒20 cm depth) collected from 15 agricultural fields, including two sites with different carbon input strategies, in Zhejiang Province, China, were separated in water-dispersible nanocolloids (0.6-25 nm), fine colloids (25-160 nm), and medium colloids (160-500 nm). Three levels of fine-colloidal P content (3500-6000, 900-2500, and 500-650 µg kg-1) were identified at the regional scale. The nanocolloidal fraction correlated with OC and Ca, and the fine colloidal fraction with OC, Si, Al and Fe. Significant linear relationships existed between colloidal P and OC, Si, Al, Fe, Ca, and for nano-colloidal P with Ca. The organic carbon controlled colloidal P saturation, which in turn affected the P carrier ability of colloids. Field scale organic carbon inputs did not change the overall pattern in size fractions of water dispersible colloids. However, they significantly affected the peak concentration in each of the nano-, fine- and medium-colloidal P concentrations. Application of chemical fertilizer with carbon-based solid manure and/or modified biochar reduced the soil nano-, fine- and medium-colloidal P content by 30-40%; however, application of chemical fertilizer with biogas slurry boosted colloidal P formation. The findings of this thesis provided a deep and novel understanding of the forms and composition of colloidal P in agricultural soils and highlighted their spatial regulation by soil characteristics and carbon inputs. In conclusion, the type of terrestrial ecosystem (forest, savanna and agriculture) is the determinant of P content, size distribution and composition of soil colloids. Management (grazing and fertilizer input) exhibited limited effects on colloidal P content and composition, however, no obvious impact on the distribution of P between different sized fractions. Meanwhile, this thesis also pointed out that soil pH, organic matter and Ca play crucial roles in P binding to soil colloids. These results provided a deeper insight into the natural soil colloids related processes in different terrestrial ecosystems.