Field experiments were conducted in years 2014–2018 on the lignite post-mining area close to Kazimierz Biskupi province Wielkopolska, Poland. The research was financially supported by European Comission LIFE + grant LIFE11ENV/PL/445 and by National Fund for Environmental Protection and Water Management in Warsaw granted to the Institute of Natural Fibres and Medicinal Plants. Before agronomic treatments could be conducted in selected experimental field, they had to be cleared of volunteer weeds and bushes as well as stones. The number of stones made initially the work on the field impossible. After removing the stones, agrotechnical works were carried out, which included disc-harrowing of the field and plowing followed by pre-sowing tillage and application of lime in the dose of 0.25 tons/ha. Liming of the field was carried out only in the first year of the project. Every year mineral fertilization was carried out in accordance with the fertilization plan: nitrogen fertilization at a dose of 150 kg N/ha, phosphorus fertilization at a dose of 150 kg P2O5/ha and potassium fertilization at the dose of 215 kg K/ha. Phosphorus and potassium fertilizers were applied in autumn and nitrogen in spring before sowing of hemp. Every year in October hemp was mowed. Mowing was conducted using a mower pulled by the tractor, equipped with three knives cutting the stem into three sections. This facilitated later plowing the biomass down. Working width of the mower was 4 m and it was able to mow 3− 4 ha/h. In November, plant residues were plowed with 30 cm deep plowing. The costs of all agronomic treatments were estimated at 660 euro per hectar per year. Samples for soil tests were collected annually before the commencement of spring agrotechnical treatments with the use of a soil auger. The field was divided into four sections, five samples were taken from each section. Laboratory tests were performed using accepted Polish standards . Phosphorus and potassium were extracted from soil using Egner-Riehm method and then phosphorus was determined by spectrophotometric method and potassium by flame photometry .
Magnesium was extracted with 0.0125 M CaCl2 solution and determined by atomic absorption spectrometry . The soil pH was determined in 1 N KCl. Micronutrients were extracted with 1 N HCl and manganese, zinc and copper were determined by atomic absorption spectrometry while boron by spectrophotometry . The presented weather pattern data is from the Weather Station in Konin . In the years of experiment the average temperatures during hemp growth season differed only a little with the exception of 2018 – the warmest year in the period of plants emergency. Precipitation in 2017 was significantly higher than in the remaining years of the experiment. Especially June and July 2017 were characterized by a lot of rainfall. The reclaimed layer was very wet due to its limited water permeability. Frequent rainfall had largely suffocated the cultivated hemp. Cannabis grow system plants died in many places due to the lack of oxygen in the soil layer, which was displaced by water . The height of the plants often did not exceed 1 m at that year. Such unfavorable weather conditions led to a reduction in the yield of plants. The results were evaluated statistically with the R statistical software version 4.0.5 . ANOVA tests were used to test main effects of recultivation year. The data from all years were combined and analysed post hoc with Fisher’s LSD test from agricolae package at a significance level α = 0.05. Agrotechnical procedures related to reclamation of the experimental field started in 2013. They consisted of clearing the field of stones, sowing lime and PK fertilization. The aforementioned treatments were designed to initiate the plot reconstruction and preparation for the planned vegetation treatment. The soil depletion with previously conducted industrial activity resulted in low hemp yielding in the first year of the experiment. The average hemp biomass yield was only 1.6 tons/ha . Already in the second year of the project, the annual incorporation of biomass from the first year led to the activation of the soil and a jump of the yield of hemp biomass to 5.3 tons/ha , followed by 5.9 tons/ha in 2016. The highest yield of 6.3 tons/ha was obtained in the last year of experiment. This growth was also influenced by favorable weather conditions favoring the development of the plants in 2018. The increase of the hemp yield was the result of the increase of the plants heigh and the straw width . In the first year of the experiment the average plants heigh was only 56 cm, while on the second year it was higher by 236 % and in 2016 reached the average height of 193 cm.
The straw width on the second and third year of experiment was 3 times bigger than in 2014 . In 2017, hemp crop suffered due to long-lasting intense rainfall. A large amount of precipitation in the period immediately after emergence led to the loss of many plants. It was reported that too much available moisture can limit production or cause failure, particularly in low lying and poorly drained fields . In the case of the reclaimed minesoil, due to the compact, impermeable recultivated layer, even slight rainfall led to the formation of large water stalls . The standing water displaced oxygen from the soil which led to the suffocation of many plants. This lead to the low hemp yield of 3,5 tons/ha and low and thin plants. Every year, the hemp biomass obtained was plowed in, enriching the soil with nutrients. This affected the level of humus in the reclaimed soil. The 2014 yield after plowing in had an impact on the content of nutrients in the soil, which was tested before the commencement of agrotechnical treatments in 2015. The same applies to all years of research. The annual incorporation of plant biomass resulted in a gradual increase in the level of humus in the reclaimed soil . The year 2014 is treated as the starting year in the conducted experiment. In 2015 we did not observe any increase of the organic matter content in the soil and it remained on a very low level of 0,88 %. It was due to the relatively small amount of plant biomass received and plowed in in 2014. Between 2015 and 2016 the increase of the OM was observed as we noted 10 % increase but is was not statistically significant. A statistically significant increase was observed in 2017. The soil survey carried out in 2017 showed a significant increase in the level of humus in the soil also in relation to the initial data from 0,87 % to 2.44 %. In the last year of experiment in 2018 we did not observe the further increase of the OM content but the deviations of the measurements decreased. In this year we did not note the large differences between samles analyzed. It is widely agreed that successful reclamation of lignite mine sites, especially when the topsoil has been removed, depends on the rapid formation of a surface horizon rich in organic matter Humic substances improve the physical properties of mine soils by favouring aggregation, which facilitates aeration and water transport. They also improve the chemical properties of soil by providing buffering capacity and increasing surface area and ion exchange capacity. As a result soils on reclamation sites can become progressively more productive with time . It the case of the presented survey we have observed relatively fast building up of the soil organic matter. Already after four years in the top layer of the studied minesoil the OM reached the level observed in the undisturbed agricultural soils of the region.
It is in line with the other authors reporting that reclaimed minesoils develop recognizable horizonation relatively quickly and effectively sequester carbon . It was reported that a distinct horizon up to 15 cm thick can develop during first 5 years of reclamation and is distinguished from subsoil by the accumulation of SOM, loose soils due to root growth, and soil structure development . The potential of organic matter sequestration in minesoils depends on biomass productivity but also on root development in the subsoil and changes in minesoil resulting of their development . Hemp grown in the optimal soil condition has a well-developed taproot system, growing into the soil to a depth of about 2 m. Duringthe course of the experiment we have observed the changes of hemp root architecture . In the first years the typical tap root was hardly visible. The lateral roots were numerous, but small and thin and outgrown rather the top layer of the minesoil. In later years, the plants began to develop a tap roots, typical for this species, which penetrated well into the soil. The root was still thinner compared to the plants grown on agricultural soils, but the root system looked typical for cannabis. Roots provide a path for movement of carbon and energy to the deeper horizons of the recultivated minesoil. Therefore, cannabis grow lights root production have a direct impact on the amount of organic matter present in the soil . The fine root contributions to SOC range from 33 to 67 percent in forest ecosystems . Balesdent and Balabane calculated that corn roots incorporated 58 percent more carbon than combined incorporation by leaves and stalks. From this perspective, the use of hemp, capable of producing a large amount of above ground biomass and at the same time developing a strong and deeply growing root system, for the rehabilitation of areas poor in organic matter, seems to be justified as deep-rooted plant species have the potential of increasing SOC sequestration by transferring more OM into deeper horizons. In many cases reported in the literature the minesoils formed after lignite mining suffer with low or extremly low pH. It was not observed in our case. During the course of experiment the pH of the minesoil remained alkaline, close to neutral . We have observed slight increase of the soil pH value during the years. Soil pH is one of the main factors determining soil fertility and it may be affected by crop and residue management. Reports on the effect of organic matter addition on soil pH have been contradictory. Some authors have suggested that accumulation of fresh organic matter may be one of the causes of soil acidification while many other reported soil pH increase after OM incorporation .
Our observations are in line with the latter reports. The the exception from this trend was observed in the second year when pH dropped from 7.7 in 2014 to 7.4. This drop was due to the lack of the soil structure typical for the minesoil at the begining of rehabilitation process, resulting with the lack of oxygen in the top layer. These together with the incorporation of the initial amount of the organic matter from the previous year could lead to the anaerobic microbial processes in the soil, production of organic acids and lowering the soil pH. The amount of all four tested macroelements increased during the course of the minesoil reclamation experiment however, the content increase dynamics was different in the case of the analyzed elements . The content of phosphorus and, to some extent, also potassium increased in proportion to the increase in the content of organic matter . At the same time, the magnesium content increased linearly in each year of the experiment, and the changes in the manganese content corresponded to the changes in the soil pH . The content of phosphorus in the tested soil increased with the reclamation carried out. This increase was statistically significant. The level of phosphorus in the reclaimed soil was below the norm, ranging from 101 to 150 mg kg of typical agricultural soil. During first two years it was extremely low . In the third year phosphorus level increased by 39 % and in 2017 by next 42 % reaching 117 mg kg. In the last year of the experiment the phosphorus content did not increase anymore but in the last two years the soil reached the minimal accepted level of 100 mg kg.