Spatial and temporal variability of soil health indicators in Northern German apple orchard soils. A space-for-time approach.

The Altes Land region in Northern Germany is the largest fruit-producing region in Germany. Here, on diked and drained freshwater tidal marsh soils of the Elbe River valley, century-old farming traditions combined with modern production methods developed into numerous different land management strategies. Soil health indicator values (e.g., bulk density, soil organic carbon quantity) display spatial and temporal variability on both field and farm scales. Assessing changes of soil health indicators remains labour-intensive, and long-term datasets usually do not sufficiently reflect small-scale heterogeneity. Attempts of monetizing soil health improvements (e.g. by carbon credits, fiscal rewards for biodiversity) require a thorough and site-specific understanding of soil health indicator development over time.

Separating short-term management-induced changes and long-term trends of perennially caused changes of soil health (e.g. carbon accumulation, soil-structure formation) involve costly long-term monitoring, but cheaper sampling snapshots might overlook long-term effects of innovative approaches of farming methods on soil health. Overcoming budgetary hurdles, we aim to achieve long-term soil health monitoring cooperation with local fruit-farmers as co-scientists. Such a transparent and transdisciplinary joint-ownership monitoring approach potentially increases acceptance of required adaptation measures and allows for reliable, robust, long-term assessment of soil health situations. For this, we initialized a soil data exchange network with interested local fruit-farmers. 

While still in the warm-up phase, we already investigated the effect of tree-planting age on selected soil health indicators, namely bulk density, total organic carbon content, and CN ratio at two differently managed apple orchards (O1 and O2) in the Northern-German Altes Land. To overcome the lack of long-term observational data, a space-for-time sampling approach was designed. The sampling design was developed in accordance with farm managers and carbon-cycling modellers. Orchard parcels were selected that represent the same management method but different tree-planting ages, assuming an accumulation of perennial management effects on soil development over time. Trees were grouped into tree-planting age categories 1 year, 10-12 years, and 20-22 years.

All three soil properties show age-dependent trends and variability across time (tree age) and space (management type). Bulk density is significantly different between planting ages 1, 10-12, and 20-22 years on O1 (Medians in g cm^-3 of 1.41, 1.26, and 0.98, respectively), but not significant on O2 (Medians in g cm^-3 of 1.29, 1.12, and 1.04, respectively). Differences in loss-on-ignition is not significant on O1 (Medians in percentages of 6.2, 7.7, and 8.1, respectively) but significant on O2 (Medians in percentages of 6.6, 6.9, and 9.0, respectively). Total organic carbon is significantly different between tree-planting ages on O1 and O2 (Medians in percentages 2.3, 3.4, and 3.5, respectively in O1, and 2.9*⁾, 3.2, and 4.1, respectively in O2); calculated with Kruskal-Wallis test, n=9 (*⁾ n=8), p<0.05.

Our first results make a strong case for site-specific long-term soil monitoring that considers temporal and spatial land-management variability. Take home message: a careful consideration of temporal and spatial management variations, as well as social and community-related aspects of land management, is advisable for soil health assessment studies in orchards.