Effects of crop rotation and organic fertilization on microbial biomass and enzyme kinetics in a long-term stockless/veganic field experiment

We need to radically transform our food and agriculture system to stay within the planetary boundaries. Sustainably increasing plant production while substantially reducing animal production unveils the need of alternatives to both chemical-based conventional and animal-based organic agriculture. In stockless farming systems, legume–grass mixtures provide no direct economic return; therefore, their proportion in crop rotations is often reduced. As a result, stockless farming is often considered challenging in terms of long-term soil fertility and plant nutrition, although research on this topic is rare. To address the specific challenges faced by veganic and stockless organic farming, a long-term field experiment was established in 2017 in Hesse, Germany, in which one veganic and two stockless organic farm systems differing in crop rotation, each combined with three fertilization systems, are compared to a mixed farm system with three livestock density levels. All systems also include an unfertilized control treatment, in which legume–grass mixtures are mulched. This experiment provides a unique basis for analyzing the long-term effects of different management and fertilization strategies on soil biogeochemical processes in detail.

Here, we aimed to study the effects of (stockless/veganic) crop rotation and organic fertilization on microbial biomass and enzyme kinetics. For this purpose, we collected soil samples from the topsoil (0-30 cm) after the harvest of potatoes in all 16 treatments in the second crop rotation of the experiment (8 years after experimental setup). We analyzed the soil microbial biomass by chloroform fumigation extraction, as well as the enzyme kinetics (V_max, K_m) of β-1,4-glucosidase (C cycle), β-1,4-N-acetylglucosaminidase, leucine aminopeptidase (N cycle), and phosphomonoesterase (P cycle) by microplate assays with fluorogenic substrates.

First results indicate that a stockless farming system, aiming at maximizing economic returns by prioritizing high-value root crops and cereals, shows a higher microbial N limitation than stockless farming systems aiming at increasing soil fertility or following veganic growing principles. Enzyme kinetics are generally more influenced by fertilization treatments than by farm types. Especially in the vegan farm type, V_max of β-1,4-glucosidase, β-1,4-N-acetylglucosaminidase, and phosphomonoesterase differed significantly between the fertilization treatments (comparing compost, cut and carry, and silage).

Taken together, microbial biomass and enzyme kinetics are dependent on fertilization type, but less on farm type/crop rotation. Stockless/veganic and mixed farms do not differ significantly at first glance, especially if the stockless/veganic farms work towards maintaining and/or increasing soil fertility.