The impact of different land uses in urban area on humus quality

Purpose

In urban areas, humus quantity and quality depend less on natural environmental factors than on anthropogenic ones. The aim of the study was to assess the impact of different land use types of urban soils on the properties of soil organic matter (SOM).

Materials and methods

Thirty-five sites involving four ways of soil use were examined: lawns, allotment gardens, fallows, and arable lands. The study was conducted in Pruszków Town in the Warsaw Agglomeration, Central Poland. Lawns and allotment gardens were located in the central part of the town, whereas fallows and arable lands were in the peripheral zones. Humus horizons, to a depth of 0–20 cm, were analyzed. Using Na-pyrophosphate extraction, we determined the soluble SOM compounds (PY), and the organic matter in the extraction residue, considered as humins (HM). In a separate extraction (with 0.05 M H₂SO₄), low molecular weight (LMW) humus compounds were determined. The quantity of humic acids (HA) precipitated during Na-pyrophosphate extraction was determined as well. A spectroscopic method (UV-Vis) was used to characterize HA properties. The absorption coefficients E4/E6 were calculated based on the results of absorbance measurements involving 465- and 665-nm wavelengths of UV-Vis light. Statistical analyses were performed to find similarities and differences between soils differently used in Pruszków.

Results and discussion

The dominant part of the humus in the studied soils were humins HMs. There were two times more HM in the central part of the town than that in the peripheral zones. The same observation was made for soluble humus compounds (PY). The amount of LMW fractions was similar in soils of all uses. The degree of humification was small and averaged about 30% for all soils. Fulvic acid (FA) concentrations predominated over HA concentrations in all soils. The least condensed HA occurred in the allotment gardens with an E4/E6 ratio of 5.7, whereas the most condensed HAs were present in soil on arable lands (E4/E6 ratio of 4.7).

Conclusions

The studies have shown that the type of land use affects humus properties. The main differences were found to be in the quantity of humus compounds. Soils from the central part of the town contained more stable (HM) and soluble (PY) compounds than soils in the outskirts of town. PY compounds were characterized by a simple structure. Fulvic acids (FA) dominated in all of the studied soils (low HA/FA ratio). A high E4/E6 ratio indicates low maturity of humic fractions with low molecular weight compounds.

     

Structural changes in humic substances after long-term fertilization of a calcareous soil with pig slurries

Pig slurries are widely used on calcareous soils in European rainfed systems. Here we assess their impact on the amount of soil organic carbon (SOC) and on the composition of humic-type substances (HTS). Seven doses of slurry (five from fattening pigs and two from sows) ranging from 1.0 to 4.8 Mg ha⁻¹ yr⁻¹ of organic matter were evaluated after a period of 12 years and compared with mineral fertilizer treatment. At the end of the last annual cropping season (September), SOC was quantified, and HTS were isolated by alkaline extraction followed by acid precipitation, and studied by visible spectroscopy (800–400 nm) and Fourier-transformed infrared spectroscopy (4000–400 cm⁻¹). Following the trend in the slurry organic matter applied rates, SOC increased from 9.5 g C kg⁻¹ (mineral treatment) to 13.8 g C kg⁻¹. This SOC increase was equivalent to c. 25.4% of the slurry organic carbon applied. The incorporation of aliphatic structures, mainly polyalkyl, from slurries into the HTS tends to modify the composition of the soil organic matter (SOM), which is reflected in a decrease in the intensity of FT-IR peaks related to aromatic structures. Despite the trend of significant increase in SOC with fattening slurries, mainly from the organic matter rate of 1.6 Mg ha⁻¹ yr⁻¹ (c. 185 kg N ha⁻¹), the composition of the HTS showed an important aliphatic enhancement. The FTIR results showed that using exclusively the relative intensities of specific peaks (alkyl, carboxyl, aromatic and amide groups) as variables for the discriminant analysis, it is possible to identify HA between different groups of soils treated with progressive levels of slurry. Although the new aliphatic components could be considered important to improve soil physical quality, after the incorporation of additional SOM, the spectroscopic characteristics of HTS in soils treated with slurries suggested a weak effect in long-term C sequestration, as the newly incorporated OC forms are not qualitatively similar to the presumably stable native SOM. These potential changes in SOC and SOM composition at field level are constrained by the maximum allowed N rates from organic origin in some agricultural systems.