Associations of Humus Content and pH of Mineral Soils with Silicate Weathering Factors and Carbon Capture

Silicate weathering is known to bind carbon. This study assesses associations between mineral soil humus content (1960-70) [Hum(us).min(.60’s)] (indicator of soil organic carbon – SOC), groundwater silicon [Si.gw], mineral (“.min”) soil parameters from 1966-70 (“68”) and 1986-90 (“88”): pH, (Ca+Mg+K), proportion of clays in fine mineral soils [Prp.(clays/fims], temperature [Temp] from 1961-90 (“(61-90)”) and 1981-2010 (“(81-10)”) by Rural Centers (RC) (N = 18). Results: Next are represented results of combined regressions, e.g. the first one shows that combined regression of [Hum.min] was explained 79.6 % by [pH. min.68;Temp.(61-90)], p< 0.001, coefficient signs (-/+). After “cf” is difference (Δ) or “E.D.” (“explained difference”) by other “[…]” parameters in the same equation. Supposed (given) inter-periodical 1.09-fold change in Si.gw and 0.82-fold change in [Hum.min], respectively Table. explained ad 45 %. Conclusion: Factors indicating silicate weathering predicted regional humus content, pH and their changes in mineral soils, best humus content. Different continuous silicate weathering possibly explains the stable relative difference in regional pH values. Increase in silicate weathering rate can reduce atmospheric and increase soil carbon Content and pH of Mineral Soils with Silicate Weathering Factors and Carbon

Conclusion: Factors indicating silicate weathering predicted regional humus content, pH and their changes in mineral soils, best humus content. Different continuous silicate weathering possibly explains the stable relative difference in regional pH values. Increase in silicate weathering rate can reduce atmospheric and increase soil carbon content.
To some extent silicate fertilizers have been known in agricultural praxis since the 19 th century [4]. The decline in soil organic carbon (SOC) from mineral cropland soils estimated by data from 1974-2009 has been ca 0.4 %/ha/a, i.e. ca 220 kg/ha/a [5]. Carbon loss has generally been associated positively with annual management practices and negatively with clay soils [5]. Biogenic amorphous silica can increase water-holding capacity of soil [6], which could resist erosion and support the maintenance of carbon balance.
Independently on universal guiding for cropland liming [7], relative regional differences in cropland soil pH-values have stayed stable for decades [8]. The aim of this study is to clarify associations of humus content and pH of mineral soils with silicate weathering

Materials and Methods
Latitude and longitude per Rural Centers (RC) -earlier ' Agricultural Advisory Centers' -have been determined, as in [9], by two-phases: first by selecting visually an approximate central commune/town of each RC in the map [10] and then via internetsearch ["GPS coordinates" and the name of the commune]. (( Table   1), includes the data for calculations). Mean annual temperatures of RC's are approximated by the same method by benefiting map [10] together with map [11] for period 1960-1990 and with map [12] for period 1981-2010. Data on groundwater silicon mean (Si.   Ostrobothnia is excluded because of its high deviation between mean and median Si.gw [18] and additionally because its soil acidity, which is obviously regulated not only by silicates and humus, but by sulfur and iron compounds [19]. In Finnish experimental stations between 1960 and 1981 agricultural humus content was reduced in 13 fields and increased in 2 fields, on an average the humus content was reduced from 7.4 to 6.0 %, i.e. annually 9 % [20]. By data in [16]

Associations of [Humus.min] with Parameters of Weathering Rate in Mineral Soils
Regressions of [Humus.min] are represented in (Figures 2-6).     Predicting of Periodical Humus.min     shows that the pH in of mineral soils is higher than that of organic soils (including mull).      Agricultural soil gets acids not only via atmospheric CO2: plant roots [26] and many micro-organisms excrete acids [27], which can promote silicate weathering and carbon capture. Humus contains even humic acids [28], with can have buffer abilities [29]. Possibly silicon/silicon compounds work as matrix for formation of humic acids [30]. By maintenance of soil humidity [6] the soluble silicates can protect soil against erosion. (Obviously too effective ditching could promote erosion and via humus loss elevate pH (Figure 9)).

Significant association of [Prp.(fims/min).68] with [Humus.
min] (Figure 2) is in concordance with Heikkinen et al. [5] who found that clay soils resisted best carbon loss or even could increase carbon binding independently on soil management [5]. Relative periodical stability in inter-regional pH values ( Figure   11) is not fully explained. Possibly plant available Si was changed inter-periodically, too.

Conclusion
Factors indicating silicate weathering predicted regional humus content, pH and their changes in mineral soils, best humus content. Different continuous silicate weathering possibly explains the stable relative difference in regional pH values. Increase in silicate weathering rate can reduce atmospheric and increase soil carbon content.