Soil microbial C:N:P stoichiometry and microbial maintenance respiration (i.e. metabolic quotient, qCO2) were monitored along a nutrient gradient in soils from a 52-year space-for-time chronosequence of reclaimed agricultural land after brown-coal mining. Land reclamation produced loess soils of initially low (0.2%) SOC. Consecutive agricultural land management led to a gradual recovery of SOC contents. Our data revealed sudden shifts in microbial stoichiometry and metabolic quotient with increasing SOC at a critical value of 1% SOC. As SOC increased, accrual rate of C into microbial biomass decreased, whereas microbial N increased. Simultaneously, metabolic quotient strongly decreased with increasing SOC until the same critical value of 1% SOC and remained at a constant low thereafter. The microbial fractions of the soil in samples containing < 1% SOC were out of stoichiometric equilibrium and were inefficient at immobilising C due to high maintenance respiration. Increasing SOC above the threshold value shifted the soil microbes towards a new equilibrium where N became growth limiting, leading to a more efficient acquisition of C. The shift in microbial N accrual was preluded by high variation in microbial biomass N in soils containing 0.5–0.9% SOC indicative of a regime shift between microbial stoichiometric equilibria. Our data may help in establishing a quantitative framework for SOC targets that, along with agricultural intensification, may better support feedback mechanisms for a sustainable accrual of C in soils.
American sycamore (Platanus occidentalis L.) was planted on bottomland and terrace sites in the Ohio River Valley region of western Kentucky to test the effects of various cultural treatments on yields over short rotation periods. Without fertilization, the bottomland site was more productive, but yields from the fertilized plots on the two sites were comparable. Fertilization significantly increased yields of all biomass components; the NP treatment did not increase yields over the N treatment. Yields of bolewood and bark were significantly increased by closer spacings. Average annual production, exclusive of leaves, for the most productive treatment combination (6 x 3 ft. spacing and N fertilization after five years on the terrace) was approximately 6 tons/acre/year on a green-weight basis. Without fertilization, the 1 x 3 ft. spacing was most productive, yielding nearly 4 tons/acre/year after either the three- or four-year rotation period on the bottom site.
<p>Every year agricultural soils lose significant amounts of nitrogen (N) over winter through N leaching and gas emissions as a result of freeze-thaw cycles. The incorporation of carbon amendments after harvest, such as crop residues or other carbon rich material, can help to promote soil microbial growth, and in doing so, immobilise N within the microbial biomass. It is still unclear which amendments are most effective at promoting microbial N immobilisation and at what time they should be incorporated into the soil to give best results.</p><p>In order to investigate this, we measured soil microbial biomass carbon (C<sub>mic</sub>) and -nitrogen (N<sub>mic</sub>) at 12 timepoints between harvest and spring in soils from an established agricultural field experiment in Kiel (Germany). We selected plots which had the same fertilisation regime and crop rotation (Faba bean-winter wheat-winter barley rotation) but differed in soil carbon amendment treatment; removal of residues (control), wheat straw, faba bean, and sawdust. &#160;In addition to microbial biomass measurements, we measured microbial nutrient limitation at each timepoint via substrate induced respiration, in order to give a qualitative indication of microbial activity in respect to growth limiting nutrients.</p><p>Our data show that there was little effect of wheat straw in comparison to the control on the microbial biomass carbon or -nitrogen, but different patterns were observed for the latter amendments. C<sub>mic</sub> generally decreased over time after harvest in all treatments, but again the decreases were less pronounced in the faba bean and sawdust treatments. N<sub>mic</sub> decreased over time after harvest in control and wheat straw treatment but increased with time in the faba bean and sawdust treatments, suggesting improved N immobilisation by the microbial biomass for these treatments. We found that all soils were nearly always N limited throughout the winter and were never P limited. However, a shift to C limitation was observed after addition of fertiliser in spring, except for in the sawdust treatment, which remained N limited despite the addition of mineral N in the field. This result suggests that sawdust has a higher potential for N immobilisation compared to the other soil amendments.</p><p>In summary, there was little difference in the microbial post-harvest dynamics between the control and wheat straw treatments but stronger effects were observed in the faba bean and saw dust treatments, which suggested improved microbial N immobilisation. Interestingly, the sawdust amendment seemed to have the highest potential for microbial N immobilisation over winter and enduring into spring.</p>
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